cp's OEIS Frontend

A278240 Least number with the prime signature of 2^n - 1.

%I A278240 #17 Nov 22 2016 21:49:09
%S A278240 1,2,2,6,2,12,2,30,6,30,6,420,2,30,30,210,2,840,2,4620,60,210,6,60060,
%T A278240 30,30,30,30030,30,60060,2,2310,210,30,210,38798760,6,30,210,1021020,
%U A278240 6,180180,30,510510,30030,210,30,446185740,6,510510,2310,510510,30,240240,30030,9699690,210,30030,6,1203362940780,2,30,60060,510510,30,19399380,6,510510,210
%N A278240 Least number with the prime signature of 2^n - 1.
%H A278240 Antti Karttunen, <a href="/A278240/b278240.txt">Table of n, a(n) for n = 1..256</a>
%F A278240 a(n) = A046523(A000225(n)).
%t A278240 Table[Times @@ MapIndexed[(Prime@ First@ #2)^#1 &, #] &@ If[Length@ # == 1 && #[[1, 1]] == 1, {0}, Reverse@ Sort@ #[[All, -1]]] &@ FactorInteger[2^n - 1], {n, 69}] (* _Michael De Vlieger_, Nov 21 2016 *)
%o A278240 (PARI)
%o A278240 allocatemem(2^30);
%o A278240 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 A278240 A278240(n) = A046523((2^n)-1);
%o A278240 for(n=1, 256, write("b278240.txt", n, " ", A278240(n)));
%o A278240 (Scheme) (define (A278240 n) (A046523 (A000225 n)))
%Y A278240 Cf. A000225, A046523.
%Y A278240 Cf. A000043 (positions of 2's), A085724 (of 6's).
%Y A278240 Cf. also A278242.
%K A278240 nonn
%O A278240 1,2
%A A278240 _Antti Karttunen_, Nov 19 2016