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A259161

Positive pentagonal numbers (A000326) that are triangular numbers (A000217) divided by 2.

5, 48510, 465793515, 4472549283020, 42945417749765025, 412361896760694487530, 3959498889750770719498535, 38019107927025003687930446040, 365059470355795195660737423378045, 3505300996337237541709397051345542550, 33657899801770684519698434826282476187555

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OFFSET

1,1

LINKS

Colin Barker, Table of n, a(n) for n = 1..251

Index entries for linear recurrences with constant coefficients, signature (9603,-9603,1).

FORMULA

G.f.: -5*x*(99*x+1) / ((x-1)*(x^2-9602*x+1)).

EXAMPLE

5 is in the sequence because 5 is the 2nd pentagonal number, and 2*5 is the 4th triangular number.

MATHEMATICA

LinearRecurrence[{9603, -9603, 1}, {5, 48510, 465793515}, 20] (* Vincenzo Librandi, Jun 20 2015 *)