OFFSET
0,2
COMMENTS
Sequence found by reading the line from 0, in the direction 0, 7, ..., in the square spiral whose vertices are the generalized enneagonal numbers A118277.
Also sequence found by reading the same line (mentioned above) in the Pythagorean spiral whose edges have length A195019 and whose vertices are the numbers A195020. This is the one of the semi-diagonals of the square spiral, which is related to the primitive Pythagorean triple [3, 4, 5]. - Omar E. Pol, Oct 13 2011
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..10000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
a(n) = 14*n^2 - 7*n = 7*A000384(n).
G.f.: -7*x*(1+3*x) / (x-1)^3. - R. J. Mathar, Sep 27 2011
PROG
(Magma) [7*n*(2*n-1): n in [0..50]]; // Vincenzo Librandi, Sep 28 2011
(PARI) a(n)=7*n*(2*n-1) \\ Charles R Greathouse IV, Sep 28 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Omar E. Pol, Sep 18 2011
STATUS
approved