OFFSET
0,12
COMMENTS
It seems to make no difference whether one requires a prime power > 1 or a prime power including 1.
While such k >= 0 exists for most odd numbers 2n+1, there are only very few even numbers of this form.
PROG
(PARI) a(n)=for(k=0, log(n=n*2+1)\log(2)+1, (/*n-2^k==1 ||*/ isprimepower(n-2^k))&&return(k)); -1
CROSSREFS
KEYWORD
sign
AUTHOR
M. F. Hasler, Nov 20 2014
STATUS
approved