OFFSET
1,2
COMMENTS
The Feit-Thompson conjecture states that given primes p and q, (p^q - 1)/(p - 1) is never divisible by (q^p - 1)/(q - 1). Assigning p = 2, the two expressions simplify to 2^q - 1 and q + 1. The former is an odd number and the latter is even, therefore there will always be a remainder when dividing the former by the latter (with the obvious exception of q = 2). This means that any counterexample to the Feit-Thompson conjecture would have to be a pair of odd primes.
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
N. M. Stephens, On the Feit-Thompson Conjecture, Mathematics of Computation, Vol. 25 (1971), p. 625.
FORMULA
EXAMPLE
a(7) = 13 because the 7th prime is 17. (2^17 - 1)/(2 - 1) gives the Mersenne prime 131071, which when divided by (17^2 - 1)/(17 - 1) = 18, leaves a remainder of 13.
MATHEMATICA
Table[Mod[2^Prime[n] - 1, Prime[n] + 1], {n, 50}]
Mod[2^#-1, #+1]&/@Prime[Range[50]] (* Harvey P. Dale, Feb 21 2012 *)
PROG
(PARI) a(n) = my(q=prime(n)); lift(Mod(2, q+1)^q - 1); \\ Michel Marcus, Jun 07 2023
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Alonso del Arte, Mar 21 2008
STATUS
approved