A272980 Least k>1 such that all powers k^e, 1 <= e <= n, are divisible by the number of their divisors, d(k^e).

2, 60, 1056, 1260, 1441440, 551350800, 42226984800, 111924212400, 11251629148359600, 284440457440339200, 582249616380374342400, 621260340677859423340800, 621260340677859423340800, 921088919608373507667359523840000000

Offset

1,1

Links

Table of n, a(n) for n=1..14.

Formula

2 / d(2) = 2 / 2 = 1 but 2^2 / d(2^2) = 4 / 3;

60 / d(60) = 60 / 12 = 5, 60^2 / d (60^2) = 3600 / 45 = 80 but 60^3 / d(60^3) = 216000 / 112 = 13500 / 7.

Maple

with(numtheory): P:= proc(q) local a, j, k, ok, n, p; a:=2;
for k from 1 to q do for n from a to q do ok:=1;
for j from 1 to k do if not type(n^j/tau(n^j), integer) then ok:=0; break; fi; od;
if ok=1 then a:=n; print(n); break; fi; od; od; end: P(10^9);

Mathematica

Table[SelectFirst[Range[2, 2*10^6], AllTrue[#^Range@ n, Divisible[#, DivisorSigma[0, #]] &] &], {n, 5}] (* Michael De Vlieger, May 12 2016, Version 10 *)

Crossrefs

Cf. A000005, A272981.

Sequence in context: A001760 A230572 A157059 * A222652 A227624 A199643

Adjacent sequences: A272977 A272978 A272979 * A272981 A272982 A272983

Keyword

nonn

Author

Paolo P. Lava, May 12 2016

Extensions

a(6)-a(14) from Giovanni Resta, May 12 2016

Status

approved