

A073610


Number of primes of the form np where p is a prime.


17



0, 0, 0, 1, 2, 1, 2, 2, 2, 3, 0, 2, 2, 3, 2, 4, 0, 4, 2, 4, 2, 5, 0, 6, 2, 5, 0, 4, 0, 6, 2, 4, 2, 7, 0, 8, 0, 3, 2, 6, 0, 8, 2, 6, 2, 7, 0, 10, 2, 8, 0, 6, 0, 10, 2, 6, 0, 7, 0, 12, 2, 5, 2, 10, 0, 12, 0, 4, 2, 10, 0, 12, 2, 9, 2, 10, 0, 14, 0, 8, 2, 9, 0, 16, 2, 9, 0, 8, 0, 18, 2, 8, 0, 9, 0, 14, 0, 6
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OFFSET

1,5


COMMENTS

a(p) = 2 if p2 is a prime else a(p) = 0. If n = 2p, p is a prime then a(n) is odd else a(n) is even. As p is counted only once and if q and nq both are prime then the count is increased by 2. ( Analogous to the fact that perfect squares have odd number of divisors).
a(2k+1) = 2 if (2k1) is prime, else a(2k+1)=0 (for any k). This sequence can be used to redescribe a couple of conjectures: the Goldbach conjecture == a(2n) > 0 for all n>=2; twin primes conjecture == for any n, there is a prime p>n s.t. a(p)>0.
Number of ordered ways of writing n as the sum of two primes.


LINKS

T. D. Noe, Table of n, a(n) for n=1..10000


FORMULA

G.f.: (Sum_{k>0} x^prime(k))^2.  Vladeta Jovovic, Mar 12 2005
Convolution of "a(n)=1 if n prime, 0 otherwise" (A010051) with itself.  Graeme McRae, Jul 18 2006


EXAMPLE

a(16) = 4 as there are 4 primes 3,5,11 and 13 such that 163,165,1611and 1613 are primes.


MAPLE

for i from 1 to 500 do a[i] := 0:j := 1:while(ithprime(j)<i) do if(isprime(iithprime(j))=true) then a[i] := a[i]+1:fi:j := j+1:od:od:seq(a[k], k=1..500);


MATHEMATICA

nn=20; a[x]:=Sum[x^i, {i, Table[Prime[n], {n, 1, nn}]}]; Drop[CoefficientList[a[x]^2, x], 1] (* Geoffrey Critzer, Nov 22 2012 *)


PROG

(PARI) Vec(sum(i=1, 100, x^prime(i), O(x^prime(101)))^2) \\ Charles R Greathouse IV, Jan 21 2015


CROSSREFS

Cf. A061358, A065577, A107318, A098238
Sequence in context: A094876 A144159 A318662 * A285797 A131840 A179752
Adjacent sequences: A073607 A073608 A073609 * A073611 A073612 A073613


KEYWORD

nonn


AUTHOR

Amarnath Murthy, Aug 05 2002


EXTENSIONS

Corrected and extended by Vladeta Jovovic and Sascha Kurz, Aug 06 2002


STATUS

approved



