2,1

Table of n, a(n) for n=2..72.

2*5 = 10 can be expressed as the sum of two primes in two ways: 3+7 and 5+5, so a(5) = 7+5 = 12.

with(numtheory); a:=n-> sum( (2*n-i)*( ((pi(i) - pi(i-1)) * (pi(2*n-i) - pi(2*n-i-1))) ), i = 1..n ); seq(a(k), k=1..100); # Wesley Ivan Hurt, Jan 20 2013

Cf. A045917, A185297, A186201.

Sequence in context: A162253 A321855 A112978 * A158936 A271227 A293696

Adjacent sequences: A187126 A187127 A187128 * A187130 A187131 A187132

nonn

N. J. A. Sloane, Mar 11 2011

approved