%I #16 Jun 11 2021 20:50:26
%S 1,1,2,8,41672,378916495683075745757412513402693048
%N a(n) = Sum_{k=0..n-1} binomial(a(k)^2, k).
%H G. C. Greubel, <a href="/A007753/b007753.txt">Table of n, a(n) for n = 0..6</a>
%p a:= proc(n) option remember;
%p if n=0 then 1
%p else add(binomial(a(j)^2, j), j=0..n-1)
%p fi; end:
%p seq(a(n), n=0..6); # _G. C. Greubel_, Mar 04 2020
%t a[n_]:= a[n]= If[n==0, 1, Sum[Binomial[a[j]^2, j], {j,0,n-1}] ]; Table[a[n], {n, 0, 6}] (* _G. C. Greubel_, Mar 04 2020 *)
%o (Sage)
%o @CachedFunction
%o def a(n):
%o if (n==0): return 1
%o else: return sum(binomial(a(j)^2, j) for j in (0..n-1))
%o [a(n) for n in (0..6)] # _G. C. Greubel_, Mar 04 2020
%K easy,nonn
%O 0,3
%A Barry Brunson (bbrunson(AT)wku.edu)