OFFSET
0,2
COMMENTS
Sequence found by reading the line from 0, in the direction 0, 7,... and the line from 15, in the direction 15, 46,..., in the square spiral whose vertices are the triangular numbers A000217.
LINKS
Index entries for linear recurrences with constant coefficients, signature (2,0,-2,1).
FORMULA
Array read by rows: row n gives 8*n^2 + 7n, 8*(n+1)^2 - (n+1).
a(n) = (3-3*(-1)^n+14*n+8*n^2)/4. a(n) = 2*a(n-1)-2*a(n-3)+a(n-4). G.f.: x*(7+x)/((1-x)^3*(1+x)). [Colin Barker, Jul 22 2012]
EXAMPLE
Array begins:
0, 7
15, 30
46, 69
93, 124
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Omar E. Pol, May 03 2008
STATUS
approved