1,4

As n increases sum(a(i)) for i=1 to n tends to e^gamma/2*n*(n+1)/2, notice that n*(n+1)/2 is triangular number. [From Pierre CAMI, Oct 16 2008]

Pierre CAMI, Table of n, a(n) for n = 1..500

2*(2+1)-1=5 prime so a(1)=1 6*(6+1)-1=41 prime so a(2)=1 30*(31)-1=929 prime so a(3)=1.

Cf. A144947

Sequence in context: A225848 A165889 A087127 * A260374 A157109 A185814

Adjacent sequences: A144943 A144944 A144945 * A144947 A144948 A144949

nonn

Pierre CAMI, Sep 26 2008, Oct 16 2008

approved