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A131903 Integers x such that d(k)=d(x) for some 0<k<x, where d=A000005 is the number of divisors. 2
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 (list; graph; refs; listen; history; text; internal format)
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: A217129 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

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Last modified December 9 03:27 EST 2019. Contains 329872 sequences. (Running on oeis4.)