login
a(1) = 1; a(n) = Sum_{k=1..n-1} ceiling(n/k) * a(k).
0

%I #8 Mar 25 2020 11:35:45

%S 1,2,7,22,69,208,634,1903,5734,17210,51702,155107,465561,1396684,

%T 4190689,12572144,37718360,113155081,339471195,1018413586,3055258062,

%U 9165774828,27497376189,82492128568,247476542954,742429628932,2227289352360,6681868062822,20045605585809

%N a(1) = 1; a(n) = Sum_{k=1..n-1} ceiling(n/k) * a(k).

%F G.f. A(x) satisfies: A(x) = x * (1 + (1/(1 - x)) * (A(x) + Sum_{k>=1} A(x^k))).

%F a(n) ~ c * 3^n, where c = 0.292080665386646518390576592052254840432101999262173908555857806023213143845... - _Vaclav Kotesovec_, Mar 25 2020

%t a[1] = 1; a[n_] := a[n] = Sum[Ceiling[n/k] a[k], {k, 1, n - 1}]; Table[a[n], {n, 1, 29}]

%t terms = 29; A[_] = 0; Do[A[x_] = x (1 + (1/(1 - x)) (A[x] + Sum[A[x^k], {k, 1, terms}])) + O[x]^(terms + 1) // Normal, terms + 1]; CoefficientList[A[x], x] // Rest

%Y Cf. A006590, A014668, A126656, A332846.

%K nonn

%O 1,2

%A _Ilya Gutkovskiy_, Mar 24 2020