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A251895
Numbers n such that the sum of the octagonal numbers N(n) and N(n+1) is equal to another octagonal number.
2
0, 30, 368, 34814, 424864, 40175518, 490292880, 46362513150, 565797558848, 53502299999774, 652929892617904, 61741607837226238, 753480530283502560, 71249761941859079070, 869515879017269336528, 82222163539297540020734, 1003420570905398530850944
OFFSET
1,2
COMMENTS
Also nonnegative integers x in the solutions to 12*x^2-6*y^2+4*x+4*y+2 = 0, the corresponding values of y being A251896.
FORMULA
a(n) = a(n-1)+1154*a(n-2)-1154*a(n-3)-a(n-4)+a(n-5).
G.f.: 2*x^2*(x^3+87*x^2-169*x-15) / ((x-1)*(x^2-34*x+1)*(x^2+34*x+1)).
EXAMPLE
30 is in the sequence because N(30)+N(31) = 2640+2821 = 5461 = N(43).
PROG
(PARI) concat(0, Vec(2*x^2*(x^3+87*x^2-169*x-15)/((x-1)*(x^2-34*x+1)*(x^2+34*x+1)) + O(x^100)))
CROSSREFS
Sequence in context: A058837 A042748 A202074 * A022690 A224332 A280478
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Dec 10 2014
STATUS
approved