A131903 Integers x such that d(k)=d(x) for some 0A000005 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

Author

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

Keywords

• easy
• nonn

Comments

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

Examples

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

Crossrefs

Cf. A069822, A131902-A131908.

Programs

Mathematica

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

PARI

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

Formula

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

Extensions

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

Edited by Danny Rorabaugh, Apr 03 2015