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%I #17 Feb 12 2024 06:36:55
%S 1,43,1366,38410,1010815,25515421,625878268,15034586596,355440320845,
%T 8298240786655,191776931546866,4395106938053518,100020864586079851,
%U 2262634152933752305,50921433140756382520,1140878530467983299336
%N a(1)=1, a(n) = n*21^(n-1) + a(n-1).
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (29,-224,196).
%F G.f.: x/((1 - x)*(1 - 21*x)^2). - _Stefano Spezia_, Mar 11 2020
%o (PARI) a(n) = (1+21^n*(20*n-1))/400; \\ _Jinyuan Wang_, Mar 11 2020
%K nonn,easy
%O 1,2
%A _Olivier Gérard_