Invitation: celebrating 50 years of OEIS, 250000 sequences, and Sloane's 75th, there will be a conference at DIMACS, Rutgers, Oct 9-10 2014.

1,4

Table of n, a(n) for n=1..81.

a(3*n-1) = eulerphi(3*n)/2 - 1.

(PARI) {a(n)=local(M=matrix(n+2, n+2, r, c, if(r>=c, if(gcd(r-c+1, c)==1, 1, 0)))^2); M[n, 3]}

Cf. A118233.

Sequence in context: A100949 A152164 A110493 * A152039 A180196 A132623

Adjacent sequences: A118231 A118232 A118233 * A118235 A118236 A118237

nonn

Leroy Quet, Paul D. Hanna, Apr 16 2006

approved