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A139423 a(1)=1, a(2)=2. For n >= 3, a(n) = a(n-1) + d(a(n-1)) + d(a(n-2)), where d(m) is the number of positive divisors of m.

%I A139423 #12 Feb 06 2019 00:19:59
%S A139423 1,2,5,9,14,21,29,35,41,47,51,57,65,73,79,83,87,93,101,107,111,117,
%T A139423 127,135,145,157,163,167,171,179,187,193,199,203,209,217,225,238,255,
%U A139423 271,281,285,295,307,313,317,321,327,335,343
%N A139423 a(1)=1, a(2)=2. For n >= 3, a(n) = a(n-1) + d(a(n-1)) + d(a(n-2)), where d(m) is the number of positive divisors of m.
%p A139423 with(numtheory): a[1]:=1: a[2]:=2: for n from 3 to 50 do a[n]:=a[n-1]+tau(a[n-1])+tau(a[n-2]) end do: seq(a[n],n=1..50); # _Emeric Deutsch_, Apr 30 2008
%t A139423 Join[{a = 1, b = 2}, Table[c = b + Total[DivisorSigma[0, {a, b}]]; a = b; b = c; c, {n, 48}]] (* _Jayanta Basu_, Jun 06 2013 *)
%Y A139423 Cf. A139422, A064491.
%K A139423 nonn
%O A139423 1,2
%A A139423 _Leroy Quet_, Apr 21 2008
%E A139423 More terms from _Emeric Deutsch_, Apr 30 2008