Primes Propaganda

Number Theory Level pending

If n n is a positive integer and p p is an odd prime , then find the number of pairs ( n , p ) (n,p) of the equation n 3 + n 2 + n + 1 = p 6 . n^3+n^2+n+1=p^6 .

0 Infinitely many solutions 6 1 2 4

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2 solutions

Pianate Nate
Jan 2, 2017

n^3+n^2+n+1 = p^6

n^3+n^2+n = p^6 - 1

n^2 x (n+1) = (p^3+1) (p^3-1)

Notice: From the LHS, if we input some values of n, we always obtain products having the last digit (Either 0, 2 or 6)

From the RHS, however, when we input some values of p, where p is odd prime, we always obtain digits other than (0, 2 or 6)

Hence, there is no solution.

LHS is okk but why RHS will always have other than 0,2,6 as last digit ?

Kushal Bose - 4 years, 5 months ago
Kazem Sepehrinia
Apr 21, 2017

Rewrite the equation as ( n + 1 ) ( n 2 + 1 ) = p 6 (n+1)(n^2+1)=p^6 Note that both of the factors on the LHS are greater than 1. Also, observe that the only prime divisor of both of the factors of LHS is p p . It means that p n + 1 p|n+1 and p n 2 + 1 p|n^2+1 . So p n ( n + 1 ) ( n 2 + 1 ) = n 1 p|n(n+1)-(n^2+1)=n-1 and p n + 1 ( n 1 ) = 2 p|n+1-(n-1)=2 Therefore p = 2 p=2 and ( n + 1 ) ( n 2 + 1 ) = 64 (n+1)(n^2+1)=64 It is clear that n < 5 n<5 , but none of the possibilities work. No solution.

Nice solution.Thanks

Kushal Bose - 4 years, 1 month ago

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