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A160689 a(1)=1. a(n) = the smallest positive integer such that d(a(n)) = d(Sum_{k=1..n} a(k)), where d(m) = the number of divisors of m. 5
1, 2, 2, 2, 8, 2, 2, 8, 2, 2, 8, 2, 2, 8, 2, 21, 5, 6, 6, 15, 3, 6, 8, 6, 2, 10, 12, 6, 12, 2, 10, 22, 8, 6, 34, 6, 6, 22, 8, 6, 8, 2, 2, 6, 8, 8, 2, 6, 15, 31, 6, 2, 6, 8, 6, 2, 2, 6, 10, 2, 6, 6, 15, 13, 6, 2, 6, 8, 2, 8, 6, 10, 6, 10, 8, 8, 6, 8, 6, 10, 8, 2, 2, 10, 2, 10, 6, 2, 38, 10, 6, 10, 8, 10, 8 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Sum_{k=1..n} a(k) = A160690(n).

d(a(n)) = d(A160690(n)) = A160691(n).

LINKS

Michel Marcus, Table of n, a(n) for n = 1..5000

MATHEMATICA

a[1] = 1; a[n_] := a[n] = (s = Sum[a[k], {k, n-1}]; For[m = 1, DivisorSigma[0, m] != DivisorSigma[0, s + m], m++]; m); Table[a[n], {n, 95}] (* Farideh Firoozbakht, May 28 2009 *)

PROG

(PARI) lista(nn) = {k = 1; print1(k, ", "); s = k; for (n=2, nn, k = 1; while(numdiv(k) != numdiv(k+s), k++); print1(k, ", "); s += k; ); } \\ Michel Marcus, Sep 04 2017

CROSSREFS

Cf. A160690, A160691.

Sequence in context: A121258 A087421 A132697 * A179976 A137508 A055921

Adjacent sequences:  A160686 A160687 A160688 * A160690 A160691 A160692

KEYWORD

nonn

AUTHOR

Leroy Quet, May 24 2009

EXTENSIONS

More terms from Farideh Firoozbakht, May 28 2009

STATUS

approved

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Last modified July 19 11:26 EDT 2018. Contains 312775 sequences. (Running on oeis4.)