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 A279952 Number of primes with prime subscripts dividing n. 1

%I

%S 0,0,1,0,1,1,0,0,1,1,1,1,0,0,2,0,1,1,0,1,1,1,0,1,1,0,1,0,0,2,1,0,2,1,

%T 1,1,0,0,1,1,1,1,0,1,2,0,0,1,0,1,2,0,0,1,2,0,1,0,1,2,0,1,1,0,1,2,1,1,

%U 1,1,0,1,0,0,2,0,1,1,0,1,1,1,1,1,2,0,1,1,0,2,0,0,2,0,1,1,0,0,2,1,0,2,0,0,2,0,0,1,1,2,1,0,0,1,1,0,1,1,1,2

%N Number of primes with prime subscripts dividing n.

%H G. C. Greubel, <a href="/A279952/b279952.txt">Table of n, a(n) for n = 1..5000</a>

%F G.f.: Sum_{k>=1} x^prime(prime(k))/(1 - x^prime(prime(k))).

%e a(15) = 2 because 15 has 4 divisors {1,3,5,15} among which 2 divisors {3,5} are primes with prime subscripts.

%t Rest[nmax = 120; CoefficientList[Series[Sum[x^Prime[Prime[k]]/(1 - x^Prime[Prime[k]]), {k, 1, nmax}], {x, 0, nmax}], x]]

%o (PARI) x='x+O('x^120); concat([0, 0], Vec(sum(k=1, 120, x^prime(prime(k))/(1 - x^prime(prime(k)))))) \\ _Indranil Ghosh_, May 23 2017

%Y Cf. A001221, A006450.

%K nonn,easy

%O 1,15

%A _Ilya Gutkovskiy_, Dec 23 2016

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Last modified August 21 05:48 EDT 2019. Contains 326162 sequences. (Running on oeis4.)