cp's OEIS Frontend

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

%I A278242 #23 Jun 16 2025 00:00:38
%S A278242 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,
%T A278242 240,6,30,4620,6,6,420,210,2310,210,30,210,60,6,30,30030,6,30,9240,
%U A278242 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
%N A278242 Least number with the prime signature of 2^n + 1.
%H A278242 Antti Karttunen, <a href="/A278242/b278242.txt">Table of n, a(n) for n = 0..257</a>
%F A278242 a(n) = A046523(A000051(n)).
%t A278242 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 *)
%o A278242 (PARI)
%o A278242 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
%o A278242 A278242(n) = A046523((2^n)+1);
%o A278242 for(n=0, 257, write("b278242.txt", n, " ", A278242(n)));
%o A278242 (Scheme) (define (A278242 n) (A046523 (A000051 n)))
%Y A278242 Cf. A000051, A046523.
%Y A278242 Cf. A073936 (positions of 6's), A092559 (positions of 4's and 6's).
%Y A278242 Cf. also A278240.
%K A278242 nonn
%O A278242 0,1
%A A278242 _Antti Karttunen_, Nov 19 2016