A278242 Least number with the prime signature of 2^n + 1.

2, 2, 2, 4, 2, 6, 6, 6, 2, 24, 12, 6, 6, 6, 30, 60, 2, 6, 210, 6, 6, 60, 30, 6, 30, 210, 210, 240, 6, 30, 4620, 6, 6, 420, 210, 2310, 210, 30, 210, 60, 6, 30, 30030, 6, 30, 9240, 2310, 30, 30, 30, 9240, 4620, 30, 30, 30030, 4620, 30, 420, 30, 210, 210, 6, 2310, 9240, 6, 30030, 30030, 30, 60, 420, 60060, 30, 2310, 30, 2310, 60060, 210, 30030, 446185740, 6, 30

Offset: 0

Author

Antti Karttunen, Nov 19 2016

Keywords

• nonn

Links

• Antti Karttunen, Table of n, a(n) for n = 0..257

Crossrefs

Cf. A000051, A046523.

Cf. A073936 (positions of 6's), A092559 (positions of 4's and 6's).

Cf. also A278240.

Programs

Mathematica

Table[Times @@ MapIndexed[(Prime@ First@ #2)^#1 &, #] &@ If[Length@ # == 1 && #[[1, 1]] == 1, {0}, Reverse@ Sort@ #[[All, -1]]] &@ FactorInteger[ 2^n + 1], {n, 0, 80}] (* Michael De Vlieger, Nov 21 2016 *)

PARI

A046523(n) = my(f=vecsort(factor(n)[, 2], , 4), p); prod(i=1, #f, (p=nextprime(p+1))^f[i]) \\ from Charles R Greathouse IV, Aug 17 2011
A278242(n) = A046523((2^n)+1);
for(n=0, 257, write("b278242.txt", n, " ", A278242(n)));

Scheme

(define (A278242 n) (A046523 (A000051 n)))

Formula

a(n) = A046523(A000051(n)).