OFFSET
1,7
COMMENTS
A series-reduced free pure achiral multifunction (SRAM) is either (case 1) the leaf symbol "o", or (case 2) a nonempty and non-unitary expression of the form h[g, ..., g] where h and g are SRAMs. The number of positions in a SRAM is the number of brackets [...] plus the number of o's.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..500
FORMULA
a(1) = 1; a(n > 1) = Sum_{0 < k < n - 1} a(k) * Sum_{d|(n - k - 1), d < n - k - 1} a(d).
EXAMPLE
The a(10) = 7 SRAMs:
o[o[o,o],o[o,o]]
o[o,o][o,o][o,o]
o[o,o][o,o,o,o,o]
o[o,o,o][o,o,o,o]
o[o,o,o,o][o,o,o]
o[o,o,o,o,o][o,o]
o[o,o,o,o,o,o,o,o]
MATHEMATICA
a[n_]:=If[n==1, 1, Sum[a[k]*Sum[a[d], {d, Most[Divisors[n-k-1]]}], {k, n-2}]];
Array[a, 12]
PROG
(PARI) seq(n)={my(p=O(x)); for(n=1, n, p = x + p*x*sum(k=2, n-2, subst(p + O(x^(n\k+1)), x, x^k)) + O(x*x^n)); Vec(p)} \\ Andrew Howroyd, Aug 19 2018
(PARI) seq(n)={my(v=vector(n)); v[1]=1; for(n=2, #v, v[n]=sum(i=1, n-2, v[i]*sumdiv(n-i-1, d, if(d<n-i-1, v[d], 0)))); v} \\ Andrew Howroyd, Aug 19 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 09 2018
EXTENSIONS
Terms a(17) and beyond from Andrew Howroyd, Aug 19 2018
STATUS
approved