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A281664 Numbers k such that A000005(k) = A000005(A000217(k)). 3
1, 2, 6, 10, 18, 22, 30, 42, 46, 58, 66, 70, 78, 82, 102, 106, 126, 130, 138, 150, 162, 166, 178, 190, 198, 210, 222, 226, 238, 250, 262, 270, 282, 306, 310, 330, 346, 358, 366, 378, 382, 418, 430, 438, 442, 462, 466, 478, 486, 490, 498, 502, 522, 546, 562 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers k such that the number of divisors of k is equal to the number of divisors of k*(k+1)/2.

LINKS

Colin Barker, Table of n, a(n) for n = 1..1000

FORMULA

a(n) = A045326(n) - 1. - Alois P. Heinz, Jan 27 2017

MAPLE

a:= proc(n) option remember; local p; p:= a(n-1)+2;

      while irem(p, 4)<>3 do p:= nextprime(p) od; p-1

    end: a(1):=1:

seq(a(n), n=1..100);  # Alois P. Heinz, Jan 27 2017

MATHEMATICA

Select[Range@ 562, DivisorSigma[0, #] == DivisorSigma[0, PolygonalNumber@ #] &] (* Michael De Vlieger, Jan 27 2017, Version 10.4 *)

PROG

(PARI) select(n->numdiv(n)==numdiv(n*(n+1)/2), vector(1000, n, n))

CROSSREFS

Cf. A000005, A000217, A006093, A045326.

Sequence in context: A255174 A140777 A309502 * A330507 A290220 A320678

Adjacent sequences:  A281661 A281662 A281663 * A281665 A281666 A281667

KEYWORD

nonn

AUTHOR

Colin Barker, Jan 26 2017

STATUS

approved

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Last modified March 30 12:10 EDT 2020. Contains 333125 sequences. (Running on oeis4.)