A115015 Sum_{i=1..n} (gcd(7,i)=1) i.

1, 3, 6, 10, 15, 21, 21, 29, 38, 48, 59, 71, 84, 84, 99, 115, 132, 150, 169, 189, 189, 211, 234, 258, 283, 309, 336, 336, 365, 395, 426, 458, 491, 525, 525, 561, 598, 636, 675, 715, 756, 756, 799, 843, 888, 934, 981, 1029, 1029, 1079, 1130, 1182, 1235, 1289, 1344, 1344

Offset: 1

Author

N. J. A. Sloane, Feb 24 2006

Keywords

• nonn

Crossrefs

Cf. A115012.

Programs

PARI

a(n)=binomial(n+1,2)-7*binomial(n\7+1,2) /* Michael Somos, Jul 12 2006 */

PARI

a(n)=sum(i=1,n, i*(1==gcd(7,i))) /* Michael Somos, Jul 12 2006 */

Formula

a(-1-n) = a(n). - Michael Somos, Jul 12 2006