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A068978 Numbers n such that sum(d|n,tau(d)/d) is an integer where tau(x) = A000005(x). 7
1, 2, 9, 18, 105, 210, 24375, 48750, 133848, 18780741, 18780965, 37561482, 37561930, 121486365, 169028685, 242972730, 338057370, 360988056, 676114740, 1120584213, 1285201500, 1352229480, 2241168426, 2776831200, 5352575025, 5408917920, 7437262140, 10705150050 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also n such that n divides A007429(n).

Also n such that n divides A211780(n). - Jaroslav Krizek, Sep 28 2014

a(28) > 10^10. - Giovanni Resta, Jun 10 2013

a(33) > 5 * 10^10. - Hiroaki Yamanouchi, Oct 05 2014

LINKS

Hiroaki Yamanouchi, Table of n, a(n) for n = 1..32

Hiroaki Yamanouchi, Conjectured table of n, a(n) for n = 1..200

MAPLE

with(numtheory); ListA064648:=proc(q) local a, b, k, n;

for n from 1 to q do a:=[op(divisors(n))]; b:=add(sigma(a[k]), k=1..nops(a));

if type(b/n, integer) then print(n); fi; od; end: ListA064648(10^6); # Paolo P. Lava, Jun 10 2013

MATHEMATICA

t = {}; n = 0; While[n++ <= 20000000, If[Mod[Total[DivisorSigma[1, Divisors[n]]], n] == 0, AppendTo[t, n]]]; t (* Jayanta Basu, Apr 03 2013 *)

PROG

(PARI) for(n=1, 20000000, if(denominator( sumdiv(n, d, numdiv(d)/d)) ==1, print1(n, ", ")))

CROSSREFS

Sequence in context: A191520 A037421 A083423 * A006226 A109298 A297470

Adjacent sequences:  A068975 A068976 A068977 * A068979 A068980 A068981

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Apr 06 2002

EXTENSIONS

More terms from Rick L. Shepherd, Jun 23 2002

a(12)-a(27) from Giovanni Resta, Jun 10 2013

a(28) from Hiroaki Yamanouchi, Oct 05 2014

STATUS

approved

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Last modified April 22 11:46 EDT 2019. Contains 322330 sequences. (Running on oeis4.)