OFFSET
1,2
COMMENTS
Given a positive integer n we construct a unique free pure symmetric multifunction e(n) by expressing n as a power of a number that is not a perfect power to a product of prime numbers: n = rad(x)^(prime(y_1) * ... * prime(y_k)) where rad = A007916. Then e(n) = e(x)[e(y_1), ..., e(y_k)].
Every free pure symmetric multifunction (with empty expressions allowed) f with one atom and n positions has a unique e-number n such that e(n) = f, and vice versa, so this sequence is a permutation of the positive integers.
LINKS
Mathematica Reference, Orderless
EXAMPLE
Triangle begins:
1
2
3 4
5 6 8 9 16
7 10 12 13 21 25 27 32 36 64 81 128 256
Corresponding triangle of free pure symmetric multifunctions (with empty expressions allowed) begins:
o,
o[],
o[][], o[o],
o[][][], o[o][], o[o[]], o[][o], o[o,o].
MATHEMATICA
maxUsing[n_]:=If[n==1, {"o"}, Join@@Cases[Table[PR[k, n-k-1], {k, n-1}], PR[h_, g_]:>Join@@Table[Apply@@@Tuples[{maxUsing[h], Union[Sort/@Tuples[maxUsing/@p]]}], {p, IntegerPartitions[g]}]]];
radQ[n_]:=And[n>1, GCD@@FactorInteger[n][[All, 2]]==1];
Clear[rad]; rad[n_]:=rad[n]=If[n==0, 1, NestWhile[#+1&, rad[n-1]+1, Not[radQ[#]]&]];
ungo[x_?AtomQ]:=1; ungo[h_[g___]]:=rad[ungo[h]]^(Times@@Prime/@ungo/@{g});
Table[Sort[ungo/@maxUsing[n]], {n, 5}]
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
Gus Wiseman, Aug 03 2018
STATUS
approved