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A131557 Triangular numbers which are the sums of five consecutive triangular numbers. +0
2
55, 2485, 17020, 799480, 5479705 (list; graph; listen)
OFFSET

1,1

FORMULA

The subsequences with odd indices and even indices satisfy the same recurrence relations : a(n+2)=322*a(n+1)-a(n)-680 and a(n+1)=161*a(n)-340+9*(320*a(n)^2-1360*a(n)-175)^0.5. The g.f. f(z)=a(1)*z+a(2)*z^2+... is given by (55*z+2485*z^2-745*z^3-3175*z^4+10*z^5+10*z^6)/((1-z^2)*(1-322*z^2+z^4)) ; we observe that the numerator is divisible by 1+z.

EXAMPLE

a(1)=55=3+6+10+15+21

CROSSREFS

Cf. A129803.

Adjacent sequences: A131554 A131555 A131556 this_sequence A131558 A131559 A131560

Sequence in context: A053113 A012048 A020536 this_sequence A119166 A027548 A076657

KEYWORD

nonn,easy,more

AUTHOR

Richard Choulet (richardchoulet(AT)yahoo.fr), Oct 06 2007

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Last modified May 16 23:01 EDT 2008. Contains 139884 sequences.


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