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A259158
Positive triangular numbers (A000217) that are heptagonal numbers (A000566) divided by 2.
4
171, 355817826, 739874066553765, 1538466018777014327028, 3199027777735470191011289295, 6651936797966044301302839516949686, 13831784604089070782667939513452521937961, 28761287297921206463992005998270360082445758120
OFFSET
1,1
FORMULA
G.f.: -171*x*(1443*x+1) / ((x-1)*(x^2-2079362*x+1)).
EXAMPLE
171 is in the sequence because 171 is the 18th triangular number, and 2*171 is the 12th heptagonal number.
MATHEMATICA
LinearRecurrence[{2079363, -2079363, 1}, {171, 355817826, 739874066553765}, 20] (* Vincenzo Librandi, Jun 20 2015 *)
PROG
(PARI) Vec(-171*x*(1443*x+1) / ((x-1)*(x^2-2079362*x+1)) + O(x^20))
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Jun 19 2015
STATUS
approved