OFFSET
1,1
COMMENTS
Extended to a(1)-a(2) using the formula/recurrence.
LINKS
Eric Weisstein's World of Mathematics, Dipyramidal Graph
Eric Weisstein's World of Mathematics, Graph Path
Index entries for linear recurrences with constant coefficients, signature (7, -21, 35, -35, 21, -7, 1).
FORMULA
a(n) = n*(-135 + 293*n - 180*n^2 + 80*n^3 - 15*n^4 + 2*n^5)/15.
a(n) = 7*a(n-1) - 21*a(n-2) + 35*a(n-3) - 35*a(n-4) + 21*a(n-5) - 7*a(n-6) + a(n-7).
G.f.: x*(-3 - 5*x + 8*x^2 - 60*x^3 + 11*x^4 - 47*x^5)/(-1 + x)^7.
MATHEMATICA
Table[n (-135 + 293 n - 180 n^2 + 80 n^3 - 15 n^4 + 2 n^5)/15, {n, 20}]
LinearRecurrence[{7, -21, 35, -35, 21, -7, 1}, {3, 26, 111, 396, 1235, 3414, 8463}, 20]
CoefficientList[Series[(-3 - 5 x + 8 x^2 - 60 x^3 + 11 x^4 - 47 x^5)/(-1 + x)^7, {x, 0, 20}], x]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Eric W. Weisstein, May 06 2019
STATUS
approved