cp's OEIS Frontend

A129138 a(n) = number of positive divisors of n that are <= phi(n), where phi(n) = A000010(n).

%I A129138 #16 Feb 01 2019 20:38:37
%S A129138 1,1,1,2,1,2,1,3,2,2,1,4,1,2,3,4,1,4,1,4,3,2,1,6,2,2,3,4,1,5,1,5,3,2,
%T A129138 3,7,1,2,3,6,1,5,1,4,5,2,1,8,2,4,3,4,1,6,3,6,3,2,1,9,1,2,5,6,3,5,1,4,
%U A129138 3,6,1,10,1,2,5,4,3,5,1,8,4,2,1,9,3,2,3,6,1,9,3,4,3,2,3,10,1,4,5,7,1,5,1,6
%N A129138 a(n) = number of positive divisors of n that are <= phi(n), where phi(n) = A000010(n).
%H A129138 Charles R Greathouse IV, <a href="/A129138/b129138.txt">Table of n, a(n) for n = 1..10000</a>
%e A129138 phi(16) = 8. So a(16) is the number of divisors of 16 which are <= 8. There are 4 such divisors: 1, 2, 4, 8; so a(16) = 4.
%p A129138 with(numtheory): a:=proc(n) local div, ct, j: div:=divisors(n): ct:=0: for j from 1 to tau(n) do if div[j]<=phi(n) then ct:=ct+1 else ct:=ct: fi od: ct; end: seq(a(n),n=1..135); # _Emeric Deutsch_, Mar 31 2007
%t A129138 Table[Length[Select[Divisors[n], # <= EulerPhi[n] &]], {n, 104}] (* _Jayanta Basu_, May 23 2013 *)
%o A129138 (PARI) a(n)=my(p=eulerphi(n));#select(k->k<=p,divisors(n)) \\ _Charles R Greathouse IV_, Mar 05 2013
%Y A129138 Cf. A129139, A126131, A074919.
%K A129138 nonn
%O A129138 1,4
%A A129138 _Leroy Quet_, Mar 30 2007
%E A129138 More terms from _Emeric Deutsch_, Mar 31 2007