A083060 a(n) is the number of natural numbers k such that A078496(k) = prime(n), where prime(n) denotes the n-th prime.

0, 0, 1, 2, 3, 2, 3, 3, 3, 3, 5, 3, 5, 4, 3, 4, 3, 6, 4, 5, 4, 4, 5, 4, 6, 4, 4, 5, 4, 4, 7, 4, 5, 7, 5, 6, 5, 5, 7, 4, 5, 6, 4, 5, 5, 5, 5, 6, 8, 4, 7, 4, 5, 6, 5, 4, 3, 8, 8, 5, 5, 5, 7, 7, 5, 5, 9, 4, 7, 8, 8, 6, 7, 4, 5, 7, 4, 7, 7, 6, 8, 7, 5, 5, 6, 7, 6

Offset: 1

Author

Serhat Sevki Dincer (sevki(AT)ug.bilkent.edu.tr), Apr 18 2003

Keywords

• easy
• nonn

Examples

			a(3)=1 since there is only k=4 for A078496(k) = prime(3) = 5.
a(8)=3 since there is only k=13,16,18 for A078496(k) = prime(8) = 19.
		

Links

• Iain Fox, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A078496.

Programs

PARI

A078496(n) = {my(p=nextprime(n+1)); while(!isprime(2*n-p), p = nextprime(p+1)); p; }
first(n) = { my(res=vector(n)); for(x=4, prime(n), my(r=primepi(A078496(x))); if(r <= n, res[r]++)); res; } \\ Iain Fox, Nov 23 2017

Formula

For n > 2, a(n) = card({k: k > 3; A078496(k) = prime(n)}).

Extensions

a(1) and a(2) prepended by Iain Fox, Nov 23 2017

More terms from Iain Fox, Nov 23 2017