A256605 Least k such that n+1 is the n-th divisor of k.

3, 4, 20, 12, 84, 120, 360, 360, 3960, 2520, 32760, 27720, 27720, 55440, 942480, 720720, 13693680, 12252240, 12252240, 12252240, 281801520, 232792560, 1163962800, 1163962800, 3491888400, 3491888400, 101264763600, 80313433200, 2489716429200, 4658179125600

Offset

2,1

Comments

The case n = 1 is not possible because the number 2 is never the first divisor of k (1 is the first divisor).

Links

Table of n, a(n) for n=2..31.

Example

a(6) = 84 because the divisors of 84 are {1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, 84} and 7 is the 6th divisor of 84.

Maple

with(numtheory):for n from 2 to 31 do:ii:=0:for  k from 1 to 10^9 while(ii=0) do:x:=divisors(k):n1:=nops(x):if n<=n1 and x[n]=n+1 then ii:=1: printf ( "%d %d \n", n, k):else fi:od:od:

Mathematica

nn=20; t=Table[0, {nn}]; found=1; n=2; While[found<nn, n++; d=Divisors[n]; Do[If[i<=nn&&d[[i]]==i+1&&t[[i]]==0, t[[i]]=n; found++], {i, Length[d]}]]; Rest[t]

Prog

(PARI) a(n) = {k = 1; ok = 0; while (!ok, d = divisors(k); if ((#d >= n) && (d[n] == n+1), ok = 1, k++); ); k; } \\ Michel Marcus, Apr 04 2015

Crossrefs

Cf. A225562.

Sequence in context: A196198 A041561 A050214 * A237884 A256532 A051719

Adjacent sequences: A256602 A256603 A256604 * A256606 A256607 A256608

Keyword

nonn

Author

Michel Lagneau, Apr 04 2015

Status

approved