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 A070996 Numbers n whose sum of divisors and number of divisors are both triangular numbers. 1
 1, 12, 45, 98, 116, 212, 356, 531, 668, 908, 909, 2036, 2192, 3425, 3548, 3771, 3871, 4076, 4500, 4689, 4779, 5625, 5636, 6212, 6224, 8012, 8108, 8361, 8469, 8800, 9725, 9819, 10916, 11828, 12060, 12593, 13388, 15200, 18068, 20144, 21212, 22393, 23481, 23612 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Donovan Johnson, Table of n, a(n) for n = 1..1000 MATHEMATICA Select[Range[25000], AllTrue[{Sqrt[1+8*DivisorSigma[0, #]], Sqrt[ 1+ 8*DivisorSigma[ 1, #]]}, OddQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Jun 01 2019 *) PROG (PARI) c=0; for(n=1, 15891631, s=sigma(n); s2=2*s; r=sqrtint(s2); if(r*(r+1)<>s2, next); nd=numdiv(n); nd2=2*nd; r=sqrtint(nd2); if(r*(r+1)==nd2, c++; write("b070996.txt", c " " n))) /* Donovan Johnson, Mar 10 2013 */ CROSSREFS Sequence in context: A194284 A009785 A135710 * A015237 A024223 A251720 Adjacent sequences:  A070993 A070994 A070995 * A070997 A070998 A070999 KEYWORD nonn AUTHOR Jason Earls, May 18 2002 STATUS approved

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Last modified July 23 16:10 EDT 2019. Contains 325258 sequences. (Running on oeis4.)