OFFSET
1,13
COMMENTS
Fred Cohen and J. L. Selfridge showed that a(n) = -1 infinitely often.
a(n) = 0 iff n is in A045718.
LINKS
Arkadiusz Wesolowski, Table of n, a(n) for n = 1..1000
Fred Cohen and J. L. Selfridge, Not every number is the sum or difference of two prime powers, Math. Comp. 29 (1975), 79-81.
Eric Weisstein's World of Mathematics, Brier Number
FORMULA
If a(n)>0, then a(2n)=a(n)-1.
EXAMPLE
For n=7, 7*2^0-1 and 7*2^0+1 are composite, but 7*2^1-1=13 is prime, so a(7)=1.
MATHEMATICA
Table[k = 0; While[! PrimeQ[n*2^k - 1] && ! PrimeQ[n*2^k + 1], k++]; k, {n, 100}] (* T. D. Noe, Aug 29 2011 *)
CROSSREFS
KEYWORD
sign
AUTHOR
Arkadiusz Wesolowski, Aug 29 2011
STATUS
approved