A282503 - OEIS

%I #20 Feb 21 2017 20:17:29

%S 1,1,1,4,4,12,12,24,72,360,360,360,360,2520,504,1008,1008,336,336,

%T 1680,1680,18480,18480,18480,92400,1201200,10810800,10810800,10810800,

%U 10810800,10810800,21621600,21621600,367567200,52509600,52509600,52509600,997682400,997682400

%N Denominator of Sum_{i = 1..n} (if(isprime(i), 0, 1/i)).

%H Robert Israel, <a href="/A282503/b282503.txt">Table of n, a(n) for n = 1..4573</a>

%e See examples in A282501.

%p S:= 0:

%p for n from 1 to 100 do

%p   if not isprime(n) then S:= S+1/n fi;

%p   A[n]:= denom(S);

%p od:

%p seq(A[i],i=1..100); # _Robert Israel_, Feb 21 2017

%t a[n_]=Sum[If[Element[k, Primes], 0, 1/k], {k, 1, n}]; Denominator /@ a /@ Range[1, 10]

%o (PARI) lista(nn) = {sr = 0; for (n=1, nn, if (!isprime(n), sr += 1/n); print1(denominator(sr), ", "););} \\ _Michel Marcus_, Feb 18 2017

%Y Cf. A018252 (nonprime numbers), A282501 (numerators).

%K nonn,frac

%O 1,4

%A _Ralf Steiner_, Feb 16 2017

%E More terms from _Michel Marcus_, Feb 18 2017