A131903 Integers x such that d(k)=d(x) for some 0<k<x, where d=A000005 is the number of divisors.

3, 5, 7, 8, 9, 10, 11, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 61, 62, 63, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78

Offset

1,1

Comments

Complement of A007416. - T. D. Noe, Jul 26 2007

Links

Table of n, a(n) for n=1..67.

Formula

a(n) = n-th element of the set {x>0 : there exists a k with 0<k<x and d(k)=d(x)}, where d=A000005 is the number of divisors.

Example

This sequence contains 8 because 8 has |{1,2,4,8}|=4 divisors and 6<8 has |{1,2,3,6}|=4 divisors.

Mathematica

Clear[tmp]; Function[n, If[Head[ #1] === tmp, #1 = n; Unevaluated[Sequence[]], n] & [tmp[DivisorSigma[0, n]]]] /@ Range[64]

Prog

(PARI) isok(n) = {my(nd = numdiv(n)); for (k=1, n-1, if (numdiv(k) == nd, return (1)); ); }

Crossrefs

Cf. A069822, A131902-A131908.

Sequence in context: A336340 A100318 A167707 * A141114 A136443 A247459

Adjacent sequences: A131900 A131901 A131902 * A131904 A131905 A131906

Keyword

easy,nonn

Author

Peter Pein (petsie(AT)dordos.net), Jul 26 2007

Extensions

a(54)-a(67) from Michel Marcus, Apr 03 2015

Edited by Danny Rorabaugh, Apr 03 2015

Status

approved