OFFSET
1,3
COMMENTS
Numbers k such that (2^k + 1)/(2*k^2 + 1) is an integer.
a(5) > 2*10^10. - Chai Wah Wu, Dec 07 2014
MATHEMATICA
a247221[n_Integer] := Select[Range[n], Divisible[2^# + 1, 2*#^2 + 1] &]; a247221[2500000] (* Michael De Vlieger, Nov 30 2014 *)
PROG
(Magma) [n: n in [1..300000] | Denominator((2^n+1)/(2*n^2+1)) eq 1];
(PARI) for(n=0, 10^9, if(Mod(2, 2*n^2+1)^n==-1, print1(n, ", "))); \\ Joerg Arndt, Nov 30 2014
(Python)
A247221_list = [n for n in range(10**6) if pow(2, n, 2*n*n+1) == 2*n*n]
# Chai Wah Wu, Dec 07 2014
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Juri-Stepan Gerasimov, Nov 30 2014
STATUS
approved