Polynomial challenge: Show that not all the coefficients of f(x) are integers.

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SUMMARY

The discussion centers on a degree 10 polynomial \( f(x) \) with integer values \( p, q, r \) such that \( f(p)=q \), \( f(q)=r \), and \( f(r)=p \). It is established that not all coefficients of \( f(x) \) can be integers, as demonstrated through the relationship \( r-p \mid p-q \). A minor correction was noted regarding the divisibility condition, which should be \( r-p \mid p-q \) instead of \( r-q \mid p-q \). The conclusion is that the polynomial's structure inherently leads to non-integer coefficients.

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castor28
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$f(x)$ is a degree 10 polynomial such that $f(p)=q$, $f(q)=r$, $f(r)=p$, where $p$, $q$, $r$ are integers with $p<q<r$.

Show that not all the coefficients of $f(x)$ are integers.
 
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castor28 said:
$f(x)$ is a degree 10 polynomial such that $f(p)=q$, $f(q)=r$, $f(r)=p$, where $p$, $q$, $r$ are integers with $p<q<r$.

Show that not all the coefficients of $f(x)$ are integers.

Let all coefficients be integers
we have m-n divides $f(m)-f(n)$
so p- q | f(p) - f(q) | q-r

similarly
q -r | r-p
and r-p | p - q

from above as p - q | q-r | r- p | p- q (all nteger multiples) so all are same and hence a contradiction

so all coefficients cannot be integers
 
Last edited:
kaliprasad said:
Let all coefficients be integers
we have m-n divides $f(m)-f(n)$
so p- q | f(p) - f(q) | q-r

similarly
q -r | r-p
and r-q | p - q

from above as p - q | q-r | r- p | p- q (all nteger multiples) so all are same and hence a contradiction

so all coefficients cannot be integers
That is quite correct (except for a small typo: $r-q\mid p-q$ should be $r-p\mid p-q$).
Congratulations!:)
 
castor28 said:
That is quite correct (except for a small typo: $r-q\mid p-q$ should be $r-p\mid p-q$).
Congratulations!:)

Thanks castor. Corrected the same inline for the flow.
 

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