

A081591


Third row of Pascal(1,6,1) array A081581.


4



1, 15, 78, 190, 351, 561, 820, 1128, 1485, 1891, 2346, 2850, 3403, 4005, 4656, 5356, 6105, 6903, 7750, 8646, 9591, 10585, 11628, 12720, 13861, 15051, 16290, 17578, 18915, 20301, 21736, 23220, 24753, 26335, 27966, 29646, 31375, 33153, 34980
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OFFSET

0,2


COMMENTS

1. Smallest triangular number T(k) (other than the trivial adjacent ones) such that T(n) + T(k) is a square. ( T(n1) and T(n+1) are trivial triangular numbers such that T(n) +T(n1) and T(n) + T(n+1) both are squares.) 0+1 = 1, 1+15 = 16, 3+ 78= 81, 6 + 190 = 196 etc. 2. (7n+5)th triangular number.  Amarnath Murthy, Jun 20 2003


LINKS



FORMULA

a(n) = (2  21*n + 49*n^2)/2.
G.f.: (1+6*x)^2/(1x)^3.
a(n) = 3*a(n1)  3*a(n2) + a(n3); a(0)=1, a(1)=15, a(2)=78.  Harvey P. Dale, Aug 03 2012


MATHEMATICA

Table[(221n+49n^2)/2, {n, 0, 40}] (* or *) LinearRecurrence[{3, 3, 1}, {1, 15, 78}, 40] (* Harvey P. Dale, Aug 03 2012 *)


PROG



CROSSREFS



KEYWORD

easy,nonn


AUTHOR



STATUS

approved



