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 A234639 Numbers n for which sigma(sigma(sigma(n))) is odd. 2
 1, 2, 4, 16, 25, 29, 50, 64, 99, 100, 104, 116, 125, 137, 243, 279, 281, 309, 317, 343, 361, 373, 396, 408, 424, 440, 456, 461, 464, 486, 488, 500, 509, 519, 534, 538, 548, 558, 568, 585, 590, 603, 618, 632, 638, 641, 646, 686, 693, 703, 709, 718, 725, 746, 747, 783 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers such that sigma(sigma(n)) is in A028982, i.e., a square or twice a square. See A234640 for the subsequence of odd terms. See also A234638, and A234641 with further cross references. LINKS R. J. Mathar, Table of n, a(n) for n = 1..404 MAPLE n := 1 : for k from 1 to 10000 do     numtheory[sigma](k) ;     numtheory[sigma](%) ;     numtheory[sigma](%) ;     if type(%, 'odd') then         printf("%d %d\n", n, k) ;         n := n+1 ;     end if; end do: # R. J. Mathar, Oct 21 2014 PROG (PARI) for(n=1, 999, sigma(sigma(sigma(n)))%2 && print1(n", ")) CROSSREFS Sequence in context: A131475 A239363 A045541 * A275674 A225983 A298080 Adjacent sequences:  A234636 A234637 A234638 * A234640 A234641 A234642 KEYWORD nonn AUTHOR M. F. Hasler, Dec 28 2013 STATUS approved

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Last modified June 16 15:38 EDT 2019. Contains 324153 sequences. (Running on oeis4.)