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A036457 Numbers k for which exactly 5 applications of A000005 are needed to reach 2. 4
60, 72, 84, 90, 96, 108, 126, 132, 140, 150, 156, 160, 180, 198, 200, 204, 220, 224, 228, 234, 240, 252, 260, 276, 288, 294, 300, 306, 308, 315, 336, 340, 342, 348, 350, 352, 360, 364, 372, 380, 392, 396, 414, 416, 420, 432, 444, 450, 460, 468, 476, 480 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Subsequences include A030630 (numbers with 12 divisors), A030636 (numbers with 18 divisors), A030638 (numbers with 20 divisors), A137491 (numbers with 28 divisors), etc. [edited by Jon E. Schoenfield, May 12 2018]

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

FORMULA

d(d(d(d(d(a(n))))))) = 2 for all n.

A036459(a(n)) = 5. - Ivan Neretin, Jan 25 2016

EXAMPLE

a(13)=180; the successive iterates are 18, 6, 4, 3, and finally the 5th is 2;

a(3)=84; divisor numbers are 12, 6, 4, 3, and 2.

MAPLE

A036459:= proc(n) option remember;

  if n <= 2 then 0 else 1 + procname(numtheory:-tau(n)) fi

end proc:

select(A036459 = 5, [$1..1000]); # Robert Israel, Jan 25 2016

MATHEMATICA

Select[Range@ 480, Last@ # == 2 && #[[5]] != 2 &@ NestList[DivisorSigma[0, #] &, #, 5] &] (* Michael De Vlieger, Jan 26 2016 *)

PROG

(PARI) is(n)=for(i=1, 4, n=numdiv(n); if(n<3, return(0))); numdiv(n)==2 \\ Charles R Greathouse IV, Sep 17 2015

CROSSREFS

Cf. A000005, A007624, A036450, A046452, A036453, A036455, A030630.

Sequence in context: A106130 A216828 A131564 * A030630 A068350 A265712

Adjacent sequences:  A036454 A036455 A036456 * A036458 A036459 A036460

KEYWORD

nonn

AUTHOR

Labos Elemer

EXTENSIONS

New name from Robert Israel, Jan 25 2016

STATUS

approved

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Last modified April 3 04:21 EDT 2020. Contains 333195 sequences. (Running on oeis4.)