A381010 Positive integers k such that 2^(k+2) - 1 is divisible by k.

1, 7, 511, 713, 11023, 15553, 43873, 81079, 95263, 323593, 628153, 2275183, 6520633, 6955513, 7947583, 10817233, 12627943, 14223823, 15346303, 19852423, 27923663, 28529473, 29360527, 31019623, 39041863, 41007823, 79015273, 134217727, 143998193, 213444943, 227018383

Offset: 1

Author

Oisín Flynn-Connolly, Apr 10 2025

Keywords

• nonn

Comments

7 is the only prime term.

Crossrefs

Cf. A000225, A055685, A069927, A187787.

Programs

Maple

q:= k-> 0=(2&^(k+2)-1) mod k:
select(q, [$1..1000000])[];  # Alois P. Heinz, Apr 10 2025

PARI

isok(k) = Mod(2, k)^(k+2) == 1; \\ Michel Marcus, Apr 10 2025

Python

def in_sequence(n):
    return pow(2, n + 2, n) == 1 % n