0,2

Sequence found by reading the line from 1, in the direction 1, 6,..., and the same line from 1, in the direction 1, 18,..., in the square spiral whose edges have length A195013 and whose vertices are the numbers A195014. Line perpendicular to the main axis A195015 in the same spiral. - Omar E. Pol, Oct 14 2011

Index entries for linear recurrences with constant coefficients, signature (2,0,-2,1).

a(n) = n*(5*n+7)/2 + 1 - n mod 2.

a(n) = Sum_{k=1..n+1} A153125(n+1,k).

a(2*n) = A033571(n); a(2*n+1) = A153127(n).

a(n) = A000566(n+1) - n mod 2.

From Colin Barker, Jul 07 2012: (Start)

a(n) = 2*a(n-1) - 2*a(n-3) + a(n-4).

G.f.: (1+4*x+6*x^2-x^3)/((1-x)^3*(1+x)). (End)

Sum_{n>=0} 1/a(n) = 5/7 + 2*sqrt(1+2/sqrt(5))*Pi/21 + 2*sqrt(5)*log(phi)/21 + 5*log(5)/21 - 8*log(2)/21, where phi is the golden ratio (A001622). - Amiram Eldar, Aug 23 2022

LinearRecurrence[{2, 0, -2, 1}, {1, 6, 18, 33}, 50] (* Harvey P. Dale, Apr 13 2014 *)

(PARI) a(n)=n*(5*n+7)/2 + 1 - n%2 \\ Charles R Greathouse IV, Oct 07 2015

Cf. A000566, A001622, A033571, A153125, A153127.

Sequence in context: A280802 A124353 A232336 * A110671 A134078 A323148

Adjacent sequences: A153123 A153124 A153125 * A153127 A153128 A153129

nonn,easy

Reinhard Zumkeller, Dec 20 2008

approved