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

1,2

Comments

7 is the only prime term.

Links

Table of n, a(n) for n=1..31.

Maple

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

Prog

(Python)
def in_sequence(n):
    return pow(2, n + 2, n) == 1 % n
(PARI) isok(k) = Mod(2, k)^(k+2) == 1; \\ Michel Marcus, Apr 10 2025

Crossrefs

Cf. A000225, A055685, A069927, A187787.

Sequence in context: A203472 A219873 A224469 * A347907 A320980 A080975

Adjacent sequences: A381007 A381008 A381009 * A381011 A381012 A381013

Keyword

nonn

Author

Oisín Flynn-Connolly, Apr 10 2025

Status

approved