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A076456

Sum of numbers that can be written as t*n + u*(n+1) for nonnegative integers t,u in exactly three ways.

9, 93, 390, 1110, 2535, 5019, 8988, 14940, 23445, 35145, 50754, 71058, 96915, 129255, 169080, 217464, 275553, 344565, 425790, 520590, 630399, 756723, 901140, 1065300, 1250925, 1459809, 1693818, 1954890, 2245035, 2566335, 2920944, 3311088, 3739065, 4207245

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OFFSET

1,1

REFERENCES

Fred. Schuh, Vragen betreffende een onbepaalde vergelijking, Nieuw Tijdschrift voor Wiskunde, 52 (1964-1965) 193-198.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).

FORMULA

a(n) = n*(n+1)*(6*n^2+4*n-1)/2.

G.f.: 3*x*(3+16*x+5*x^2)/(1-x)^5.

MAPLE