Which monomials in $p$ and $q$ can be a probability in a finite coin experiment?

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The question generate event with $6.75space p^2q$, $20space p^3q^2$, $3.9space pq$? prompted me to think more generally about which expressions of this form can be a probability in a finite coin experiment. Since the answer turns out to be rather nice, I thought I'd post it here as an answer to the general question.



We have a coin that shows heads with probability $p$ and tails with probability $q:=1-p$, and the question is whether for given $muinmathbb R^+$ and $i,jinmathbb N$ (not both zero) there is an event in an experiment with a fixed finite number of tosses of the coin whose probability is $mu p^iq^j$ for all $pin[0,1]$.







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    The question generate event with $6.75space p^2q$, $20space p^3q^2$, $3.9space pq$? prompted me to think more generally about which expressions of this form can be a probability in a finite coin experiment. Since the answer turns out to be rather nice, I thought I'd post it here as an answer to the general question.



    We have a coin that shows heads with probability $p$ and tails with probability $q:=1-p$, and the question is whether for given $muinmathbb R^+$ and $i,jinmathbb N$ (not both zero) there is an event in an experiment with a fixed finite number of tosses of the coin whose probability is $mu p^iq^j$ for all $pin[0,1]$.







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      up vote
      1
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      1









      up vote
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      down vote

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      1





      The question generate event with $6.75space p^2q$, $20space p^3q^2$, $3.9space pq$? prompted me to think more generally about which expressions of this form can be a probability in a finite coin experiment. Since the answer turns out to be rather nice, I thought I'd post it here as an answer to the general question.



      We have a coin that shows heads with probability $p$ and tails with probability $q:=1-p$, and the question is whether for given $muinmathbb R^+$ and $i,jinmathbb N$ (not both zero) there is an event in an experiment with a fixed finite number of tosses of the coin whose probability is $mu p^iq^j$ for all $pin[0,1]$.







      share|cite|improve this question











      The question generate event with $6.75space p^2q$, $20space p^3q^2$, $3.9space pq$? prompted me to think more generally about which expressions of this form can be a probability in a finite coin experiment. Since the answer turns out to be rather nice, I thought I'd post it here as an answer to the general question.



      We have a coin that shows heads with probability $p$ and tails with probability $q:=1-p$, and the question is whether for given $muinmathbb R^+$ and $i,jinmathbb N$ (not both zero) there is an event in an experiment with a fixed finite number of tosses of the coin whose probability is $mu p^iq^j$ for all $pin[0,1]$.









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      asked Jul 16 at 21:33









      joriki

      164k10180328




      164k10180328




















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          The monomial $mu p^iq^j$ is a probability in a finite coin experiment if and only if $mu$ is an integer and $mu p^iq^jlt1$ for all $pin[0,1]$.




          First, $mu$ needs to be an integer: The probability of any event in an experiment with a fixed finite number of tosses is a polynomial in $p$ with integer coefficients; and $mu p^iq^j=mu p^i(1-p)^j$ is a polynomial in $p$ with coefficient $mu$ for $p^i$, so $mu$ must be integer for the two to be equal.



          The rest of the “only if” direction is also readily proved. Since $mu p^iq^j$ is a probability for all $p$, it must be $le1$ for all $p$. If there were $p$ such that $mu p^iq^j=1$, the event would have to include all elementary events, which implies $mu p^iq^j=1$ for all $p$, which only occurs in the trivial case $i=j=0$ that was excluded in the question.



          Now, to prove the “if” direction, consider an experiment with $i+j+l$ tosses. If we can select $mubinom lk$ elementary events with $i+k$ heads and $j+l-k$ tails for all $k=0,ldots,l$, then the probability of the event composed of all these elementary events is



          $$
          sum_k=0^lmubinom lkp^i+kq^j+l-k=mu p^iq^j(p+q)^l=mu p^iq^j;.
          $$



          There are $binomi+j+li+k$ such elementary events, so we can choose $mubinom lk$ of them if



          $$
          mubinom lklebinomi+j+li+k
          $$



          for all $k$. If we cancel factors in the three pairs of corresponding factorials, this becomes



          $$
          mu(k+i)cdots(k+1)(l-k+j)cdots(l-k+1)le(l+i+j)cdots(l+1);,
          $$



          and then dividing through by $l^i+j$ and denoting $frac klin[0,1]$ by $alpha$ leads to



          $$
          muleft(alpha+frac ilright)cdotsleft(alpha+frac1lright)left(1-alpha+frac jlright)cdotsleft(1-alpha+frac 1lright)leleft(1+fraci+jlright)cdotsleft(1+frac1lright)
          $$



          and thus to



          $$
          mualpha^i(1-alpha)^j+Oleft(frac(i+j)^2lright)le1;.
          $$



          Thus, we can satisfy this inequality for sufficiently large $l$ if $mualpha^i(1-alpha)^jlt1$ for all $alphain[0,1]$.






          share|cite|improve this answer





















          • but, if you take an event from a subset of the (finite) probability space, can't $mu$ be rational ? and even greater than one , keeping safe that $mu p^i q^j <1$ ?
            – G Cab
            Jul 16 at 21:49










          • @GCab: $mu$ can certainly be greater than $1$; in the original question that I linked to, it was $20$. But I don't understand your question about $mu$ being rational. Is there an error in my proof that it can't? Perhaps I don't understand what you mean by "taking an event from a subset".
            – joriki
            Jul 16 at 21:54










          • My doubt was about introducing order. But I got your point now, in a different way: let's say that each of all the possible binary strings of length $n$ has a probability that corresponds to the product of $(p_1p_2cdots p_m,;q_1q_2cdots q_n-m)$ in whichever order. So $p_iq_j$ is the .."granularity" of any event you can define in them, and so $mu$ must be an integer. (+1)
            – G Cab
            Jul 16 at 23:50











          • @GCab: I don't think the argument works in quite that way. As the proof here and already the example given in the linked question illustrate, you can make use of $p+q=1$ to combine monomials into lower ones; and you can also use it to artificially increase the degree of a polynomial in $p$ and $q$; so I don't see how working with $p$ and $q$ allows for such a "granularity" argument; I think you really do need to switch to a representation in terms of $p$ alone, as I did in the proof, in order to make that argument go through.
            – joriki
            Jul 17 at 3:56










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          accepted











          The monomial $mu p^iq^j$ is a probability in a finite coin experiment if and only if $mu$ is an integer and $mu p^iq^jlt1$ for all $pin[0,1]$.




          First, $mu$ needs to be an integer: The probability of any event in an experiment with a fixed finite number of tosses is a polynomial in $p$ with integer coefficients; and $mu p^iq^j=mu p^i(1-p)^j$ is a polynomial in $p$ with coefficient $mu$ for $p^i$, so $mu$ must be integer for the two to be equal.



          The rest of the “only if” direction is also readily proved. Since $mu p^iq^j$ is a probability for all $p$, it must be $le1$ for all $p$. If there were $p$ such that $mu p^iq^j=1$, the event would have to include all elementary events, which implies $mu p^iq^j=1$ for all $p$, which only occurs in the trivial case $i=j=0$ that was excluded in the question.



          Now, to prove the “if” direction, consider an experiment with $i+j+l$ tosses. If we can select $mubinom lk$ elementary events with $i+k$ heads and $j+l-k$ tails for all $k=0,ldots,l$, then the probability of the event composed of all these elementary events is



          $$
          sum_k=0^lmubinom lkp^i+kq^j+l-k=mu p^iq^j(p+q)^l=mu p^iq^j;.
          $$



          There are $binomi+j+li+k$ such elementary events, so we can choose $mubinom lk$ of them if



          $$
          mubinom lklebinomi+j+li+k
          $$



          for all $k$. If we cancel factors in the three pairs of corresponding factorials, this becomes



          $$
          mu(k+i)cdots(k+1)(l-k+j)cdots(l-k+1)le(l+i+j)cdots(l+1);,
          $$



          and then dividing through by $l^i+j$ and denoting $frac klin[0,1]$ by $alpha$ leads to



          $$
          muleft(alpha+frac ilright)cdotsleft(alpha+frac1lright)left(1-alpha+frac jlright)cdotsleft(1-alpha+frac 1lright)leleft(1+fraci+jlright)cdotsleft(1+frac1lright)
          $$



          and thus to



          $$
          mualpha^i(1-alpha)^j+Oleft(frac(i+j)^2lright)le1;.
          $$



          Thus, we can satisfy this inequality for sufficiently large $l$ if $mualpha^i(1-alpha)^jlt1$ for all $alphain[0,1]$.






          share|cite|improve this answer





















          • but, if you take an event from a subset of the (finite) probability space, can't $mu$ be rational ? and even greater than one , keeping safe that $mu p^i q^j <1$ ?
            – G Cab
            Jul 16 at 21:49










          • @GCab: $mu$ can certainly be greater than $1$; in the original question that I linked to, it was $20$. But I don't understand your question about $mu$ being rational. Is there an error in my proof that it can't? Perhaps I don't understand what you mean by "taking an event from a subset".
            – joriki
            Jul 16 at 21:54










          • My doubt was about introducing order. But I got your point now, in a different way: let's say that each of all the possible binary strings of length $n$ has a probability that corresponds to the product of $(p_1p_2cdots p_m,;q_1q_2cdots q_n-m)$ in whichever order. So $p_iq_j$ is the .."granularity" of any event you can define in them, and so $mu$ must be an integer. (+1)
            – G Cab
            Jul 16 at 23:50











          • @GCab: I don't think the argument works in quite that way. As the proof here and already the example given in the linked question illustrate, you can make use of $p+q=1$ to combine monomials into lower ones; and you can also use it to artificially increase the degree of a polynomial in $p$ and $q$; so I don't see how working with $p$ and $q$ allows for such a "granularity" argument; I think you really do need to switch to a representation in terms of $p$ alone, as I did in the proof, in order to make that argument go through.
            – joriki
            Jul 17 at 3:56














          up vote
          2
          down vote



          accepted











          The monomial $mu p^iq^j$ is a probability in a finite coin experiment if and only if $mu$ is an integer and $mu p^iq^jlt1$ for all $pin[0,1]$.




          First, $mu$ needs to be an integer: The probability of any event in an experiment with a fixed finite number of tosses is a polynomial in $p$ with integer coefficients; and $mu p^iq^j=mu p^i(1-p)^j$ is a polynomial in $p$ with coefficient $mu$ for $p^i$, so $mu$ must be integer for the two to be equal.



          The rest of the “only if” direction is also readily proved. Since $mu p^iq^j$ is a probability for all $p$, it must be $le1$ for all $p$. If there were $p$ such that $mu p^iq^j=1$, the event would have to include all elementary events, which implies $mu p^iq^j=1$ for all $p$, which only occurs in the trivial case $i=j=0$ that was excluded in the question.



          Now, to prove the “if” direction, consider an experiment with $i+j+l$ tosses. If we can select $mubinom lk$ elementary events with $i+k$ heads and $j+l-k$ tails for all $k=0,ldots,l$, then the probability of the event composed of all these elementary events is



          $$
          sum_k=0^lmubinom lkp^i+kq^j+l-k=mu p^iq^j(p+q)^l=mu p^iq^j;.
          $$



          There are $binomi+j+li+k$ such elementary events, so we can choose $mubinom lk$ of them if



          $$
          mubinom lklebinomi+j+li+k
          $$



          for all $k$. If we cancel factors in the three pairs of corresponding factorials, this becomes



          $$
          mu(k+i)cdots(k+1)(l-k+j)cdots(l-k+1)le(l+i+j)cdots(l+1);,
          $$



          and then dividing through by $l^i+j$ and denoting $frac klin[0,1]$ by $alpha$ leads to



          $$
          muleft(alpha+frac ilright)cdotsleft(alpha+frac1lright)left(1-alpha+frac jlright)cdotsleft(1-alpha+frac 1lright)leleft(1+fraci+jlright)cdotsleft(1+frac1lright)
          $$



          and thus to



          $$
          mualpha^i(1-alpha)^j+Oleft(frac(i+j)^2lright)le1;.
          $$



          Thus, we can satisfy this inequality for sufficiently large $l$ if $mualpha^i(1-alpha)^jlt1$ for all $alphain[0,1]$.






          share|cite|improve this answer





















          • but, if you take an event from a subset of the (finite) probability space, can't $mu$ be rational ? and even greater than one , keeping safe that $mu p^i q^j <1$ ?
            – G Cab
            Jul 16 at 21:49










          • @GCab: $mu$ can certainly be greater than $1$; in the original question that I linked to, it was $20$. But I don't understand your question about $mu$ being rational. Is there an error in my proof that it can't? Perhaps I don't understand what you mean by "taking an event from a subset".
            – joriki
            Jul 16 at 21:54










          • My doubt was about introducing order. But I got your point now, in a different way: let's say that each of all the possible binary strings of length $n$ has a probability that corresponds to the product of $(p_1p_2cdots p_m,;q_1q_2cdots q_n-m)$ in whichever order. So $p_iq_j$ is the .."granularity" of any event you can define in them, and so $mu$ must be an integer. (+1)
            – G Cab
            Jul 16 at 23:50











          • @GCab: I don't think the argument works in quite that way. As the proof here and already the example given in the linked question illustrate, you can make use of $p+q=1$ to combine monomials into lower ones; and you can also use it to artificially increase the degree of a polynomial in $p$ and $q$; so I don't see how working with $p$ and $q$ allows for such a "granularity" argument; I think you really do need to switch to a representation in terms of $p$ alone, as I did in the proof, in order to make that argument go through.
            – joriki
            Jul 17 at 3:56












          up vote
          2
          down vote



          accepted







          up vote
          2
          down vote



          accepted







          The monomial $mu p^iq^j$ is a probability in a finite coin experiment if and only if $mu$ is an integer and $mu p^iq^jlt1$ for all $pin[0,1]$.




          First, $mu$ needs to be an integer: The probability of any event in an experiment with a fixed finite number of tosses is a polynomial in $p$ with integer coefficients; and $mu p^iq^j=mu p^i(1-p)^j$ is a polynomial in $p$ with coefficient $mu$ for $p^i$, so $mu$ must be integer for the two to be equal.



          The rest of the “only if” direction is also readily proved. Since $mu p^iq^j$ is a probability for all $p$, it must be $le1$ for all $p$. If there were $p$ such that $mu p^iq^j=1$, the event would have to include all elementary events, which implies $mu p^iq^j=1$ for all $p$, which only occurs in the trivial case $i=j=0$ that was excluded in the question.



          Now, to prove the “if” direction, consider an experiment with $i+j+l$ tosses. If we can select $mubinom lk$ elementary events with $i+k$ heads and $j+l-k$ tails for all $k=0,ldots,l$, then the probability of the event composed of all these elementary events is



          $$
          sum_k=0^lmubinom lkp^i+kq^j+l-k=mu p^iq^j(p+q)^l=mu p^iq^j;.
          $$



          There are $binomi+j+li+k$ such elementary events, so we can choose $mubinom lk$ of them if



          $$
          mubinom lklebinomi+j+li+k
          $$



          for all $k$. If we cancel factors in the three pairs of corresponding factorials, this becomes



          $$
          mu(k+i)cdots(k+1)(l-k+j)cdots(l-k+1)le(l+i+j)cdots(l+1);,
          $$



          and then dividing through by $l^i+j$ and denoting $frac klin[0,1]$ by $alpha$ leads to



          $$
          muleft(alpha+frac ilright)cdotsleft(alpha+frac1lright)left(1-alpha+frac jlright)cdotsleft(1-alpha+frac 1lright)leleft(1+fraci+jlright)cdotsleft(1+frac1lright)
          $$



          and thus to



          $$
          mualpha^i(1-alpha)^j+Oleft(frac(i+j)^2lright)le1;.
          $$



          Thus, we can satisfy this inequality for sufficiently large $l$ if $mualpha^i(1-alpha)^jlt1$ for all $alphain[0,1]$.






          share|cite|improve this answer














          The monomial $mu p^iq^j$ is a probability in a finite coin experiment if and only if $mu$ is an integer and $mu p^iq^jlt1$ for all $pin[0,1]$.




          First, $mu$ needs to be an integer: The probability of any event in an experiment with a fixed finite number of tosses is a polynomial in $p$ with integer coefficients; and $mu p^iq^j=mu p^i(1-p)^j$ is a polynomial in $p$ with coefficient $mu$ for $p^i$, so $mu$ must be integer for the two to be equal.



          The rest of the “only if” direction is also readily proved. Since $mu p^iq^j$ is a probability for all $p$, it must be $le1$ for all $p$. If there were $p$ such that $mu p^iq^j=1$, the event would have to include all elementary events, which implies $mu p^iq^j=1$ for all $p$, which only occurs in the trivial case $i=j=0$ that was excluded in the question.



          Now, to prove the “if” direction, consider an experiment with $i+j+l$ tosses. If we can select $mubinom lk$ elementary events with $i+k$ heads and $j+l-k$ tails for all $k=0,ldots,l$, then the probability of the event composed of all these elementary events is



          $$
          sum_k=0^lmubinom lkp^i+kq^j+l-k=mu p^iq^j(p+q)^l=mu p^iq^j;.
          $$



          There are $binomi+j+li+k$ such elementary events, so we can choose $mubinom lk$ of them if



          $$
          mubinom lklebinomi+j+li+k
          $$



          for all $k$. If we cancel factors in the three pairs of corresponding factorials, this becomes



          $$
          mu(k+i)cdots(k+1)(l-k+j)cdots(l-k+1)le(l+i+j)cdots(l+1);,
          $$



          and then dividing through by $l^i+j$ and denoting $frac klin[0,1]$ by $alpha$ leads to



          $$
          muleft(alpha+frac ilright)cdotsleft(alpha+frac1lright)left(1-alpha+frac jlright)cdotsleft(1-alpha+frac 1lright)leleft(1+fraci+jlright)cdotsleft(1+frac1lright)
          $$



          and thus to



          $$
          mualpha^i(1-alpha)^j+Oleft(frac(i+j)^2lright)le1;.
          $$



          Thus, we can satisfy this inequality for sufficiently large $l$ if $mualpha^i(1-alpha)^jlt1$ for all $alphain[0,1]$.







          share|cite|improve this answer













          share|cite|improve this answer



          share|cite|improve this answer











          answered Jul 16 at 21:33









          joriki

          164k10180328




          164k10180328











          • but, if you take an event from a subset of the (finite) probability space, can't $mu$ be rational ? and even greater than one , keeping safe that $mu p^i q^j <1$ ?
            – G Cab
            Jul 16 at 21:49










          • @GCab: $mu$ can certainly be greater than $1$; in the original question that I linked to, it was $20$. But I don't understand your question about $mu$ being rational. Is there an error in my proof that it can't? Perhaps I don't understand what you mean by "taking an event from a subset".
            – joriki
            Jul 16 at 21:54










          • My doubt was about introducing order. But I got your point now, in a different way: let's say that each of all the possible binary strings of length $n$ has a probability that corresponds to the product of $(p_1p_2cdots p_m,;q_1q_2cdots q_n-m)$ in whichever order. So $p_iq_j$ is the .."granularity" of any event you can define in them, and so $mu$ must be an integer. (+1)
            – G Cab
            Jul 16 at 23:50











          • @GCab: I don't think the argument works in quite that way. As the proof here and already the example given in the linked question illustrate, you can make use of $p+q=1$ to combine monomials into lower ones; and you can also use it to artificially increase the degree of a polynomial in $p$ and $q$; so I don't see how working with $p$ and $q$ allows for such a "granularity" argument; I think you really do need to switch to a representation in terms of $p$ alone, as I did in the proof, in order to make that argument go through.
            – joriki
            Jul 17 at 3:56
















          • but, if you take an event from a subset of the (finite) probability space, can't $mu$ be rational ? and even greater than one , keeping safe that $mu p^i q^j <1$ ?
            – G Cab
            Jul 16 at 21:49










          • @GCab: $mu$ can certainly be greater than $1$; in the original question that I linked to, it was $20$. But I don't understand your question about $mu$ being rational. Is there an error in my proof that it can't? Perhaps I don't understand what you mean by "taking an event from a subset".
            – joriki
            Jul 16 at 21:54










          • My doubt was about introducing order. But I got your point now, in a different way: let's say that each of all the possible binary strings of length $n$ has a probability that corresponds to the product of $(p_1p_2cdots p_m,;q_1q_2cdots q_n-m)$ in whichever order. So $p_iq_j$ is the .."granularity" of any event you can define in them, and so $mu$ must be an integer. (+1)
            – G Cab
            Jul 16 at 23:50











          • @GCab: I don't think the argument works in quite that way. As the proof here and already the example given in the linked question illustrate, you can make use of $p+q=1$ to combine monomials into lower ones; and you can also use it to artificially increase the degree of a polynomial in $p$ and $q$; so I don't see how working with $p$ and $q$ allows for such a "granularity" argument; I think you really do need to switch to a representation in terms of $p$ alone, as I did in the proof, in order to make that argument go through.
            – joriki
            Jul 17 at 3:56















          but, if you take an event from a subset of the (finite) probability space, can't $mu$ be rational ? and even greater than one , keeping safe that $mu p^i q^j <1$ ?
          – G Cab
          Jul 16 at 21:49




          but, if you take an event from a subset of the (finite) probability space, can't $mu$ be rational ? and even greater than one , keeping safe that $mu p^i q^j <1$ ?
          – G Cab
          Jul 16 at 21:49












          @GCab: $mu$ can certainly be greater than $1$; in the original question that I linked to, it was $20$. But I don't understand your question about $mu$ being rational. Is there an error in my proof that it can't? Perhaps I don't understand what you mean by "taking an event from a subset".
          – joriki
          Jul 16 at 21:54




          @GCab: $mu$ can certainly be greater than $1$; in the original question that I linked to, it was $20$. But I don't understand your question about $mu$ being rational. Is there an error in my proof that it can't? Perhaps I don't understand what you mean by "taking an event from a subset".
          – joriki
          Jul 16 at 21:54












          My doubt was about introducing order. But I got your point now, in a different way: let's say that each of all the possible binary strings of length $n$ has a probability that corresponds to the product of $(p_1p_2cdots p_m,;q_1q_2cdots q_n-m)$ in whichever order. So $p_iq_j$ is the .."granularity" of any event you can define in them, and so $mu$ must be an integer. (+1)
          – G Cab
          Jul 16 at 23:50





          My doubt was about introducing order. But I got your point now, in a different way: let's say that each of all the possible binary strings of length $n$ has a probability that corresponds to the product of $(p_1p_2cdots p_m,;q_1q_2cdots q_n-m)$ in whichever order. So $p_iq_j$ is the .."granularity" of any event you can define in them, and so $mu$ must be an integer. (+1)
          – G Cab
          Jul 16 at 23:50













          @GCab: I don't think the argument works in quite that way. As the proof here and already the example given in the linked question illustrate, you can make use of $p+q=1$ to combine monomials into lower ones; and you can also use it to artificially increase the degree of a polynomial in $p$ and $q$; so I don't see how working with $p$ and $q$ allows for such a "granularity" argument; I think you really do need to switch to a representation in terms of $p$ alone, as I did in the proof, in order to make that argument go through.
          – joriki
          Jul 17 at 3:56




          @GCab: I don't think the argument works in quite that way. As the proof here and already the example given in the linked question illustrate, you can make use of $p+q=1$ to combine monomials into lower ones; and you can also use it to artificially increase the degree of a polynomial in $p$ and $q$; so I don't see how working with $p$ and $q$ allows for such a "granularity" argument; I think you really do need to switch to a representation in terms of $p$ alone, as I did in the proof, in order to make that argument go through.
          – joriki
          Jul 17 at 3:56












           

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