Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #11 Aug 02 2019 22:59:24
%S 1,1,3,2,1,1,3,8,1,1,3,2,1,1,2,3,4,1,1,3,2,1,12,1,3,8,1,1,3,2,1,1,2,3,
%T 4,1,1,8,3,2,1,1,3,8,1,6,1,3,2,1,1,3,4,1,6,1,3,2,1,1,2,3,8,1,1,4,3,2,
%U 1,24,1,3,4,1,1,3,2,1,1,3,8,1,1,4,3,2,1,24,1,2,3,4,1,6,1,3,2,1,1,2,3,8,1,1,3
%N If m is the n-th composite, then a(n) = gcd(k + m/k), where k is over all divisors of m.
%C Conjecture: All even numbers are terms and the only odd numbers which are terms are 1 and 3. - _Robert G. Wilson v_, Sep 08 2008
%e For n=11, 20 is the 11th composite. So we have a(11) = gcd(1+20, 2+10, 4+5, 5+4, 10+2, 20+1) = 3.
%t Composite[n_Integer] := FixedPoint[n + PrimePi@# + 1 &, n + PrimePi@n + 1]; f[n_] := Block[{m = Composite@n}, Last@ FoldList[ GCD, m!, # + m/# & /@ Divisors@m]]; Array[f, 105] (* _Robert G. Wilson v_, Sep 08 2008 *)
%Y Cf. A143771.
%K nonn
%O 1,3
%A _Leroy Quet_, Aug 31 2008
%E More terms from _Robert G. Wilson v_, Sep 08 2008