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 A139695 a(1)=1. a(n) = the smallest integer > a(n-1) such that |d(a(n)) - d(a(n-1))| = n-1, where d(m) = the number of positive divisors of m. 2
 1, 2, 6, 64, 121, 128, 131, 196, 65536, 65541, 65572, 117649, 262144, 262148, 262192, 279841, 287296, 287299, 287744, 292681, 4194304, 4194319, 4194325, 70368744177664, 2384185791015625, 2384185791015648, 2384185791085568 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(24) > 1400000000. - Robert G. Wilson v, Jun 06 2008 If n-1 >= d(a(n-1)), then d(a(n)) must be n-1 + d(a(n-1)). LINKS Ray Chandler, Table of n, a(n) for n=1..89 MAPLE A139695 := proc(n) option remember ; local a, aprev; if n = 1 then 1; else aprev := A139695(n-1) ; for a from aprev+1 do if abs(numtheory[tau](a)-numtheory[tau](aprev)) = n-1 then RETURN(a) ; fi ; od: fi ; end: for n from 1 do print(A139695(n)) ; od: # R. J. Mathar, May 04 2008 MATHEMATICA f[1] = 1; f[n_] := f[n] = Block[{da = DivisorSigma[0, f[n - 1]], k = f[n - 1] + 1}, While[ Abs[ DivisorSigma[0, k] - da] + 1 != n, k++; m = k]; k]; Do[ Print[{n, f@n}], {n, 50}] CROSSREFS Cf. A139696. Sequence in context: A145756 A030170 A082640 * A347949 A241590 A052522 Adjacent sequences: A139692 A139693 A139694 * A139696 A139697 A139698 KEYWORD nonn AUTHOR Leroy Quet, Apr 29 2008, Jun 14 2008 EXTENSIONS a(10)-a(20) from R. J. Mathar, May 04 2008 a(21)-a(23) from Robert G. Wilson v, Jun 05 2008 a(24)-a(27) from Ray Chandler, Jul 08 2009 STATUS approved

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Last modified June 15 11:51 EDT 2024. Contains 373407 sequences. (Running on oeis4.)