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b:= proc(n, m) option remember; `if`(n=0,
(2*m+1)^(m-1), m*b(n-1, m)+b(n-1, m+1))
end:
a:= n-> b(n, 0):
seq(a(n), n=0..24); # Alois P. Heinz, Jul 29 2022
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From _G.f.: Sum_{k>=0} (2*k+1)^(k-1) * x^k/Product_{j=1..k} (1 - j*x). - _Seiichi Manyama_, Nov 20 2021: (Start)
a(0) = 1; a(n) = Sum_{k=1..n} (2*k+1)^(k-1) * binomial(n-1,k-1) * a(n-k).
G.f.: Sum_{k>=0} (2*k+1)^(k-1) * x^k/Product_{j=1..k} (1 - j*x). (End)
(PARI) a(n) = if(n==0, 1, sum(k=1, n, (2*k+1)^(k-1)*binomial(n-1, k-1)*a(n-k))); \\ Seiichi Manyama, Nov 20 2021
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