OFFSET
1,2
EXAMPLE
Sequence of trees begins:
1 o
4 (oo)
14 (o(oo))
16 (oooo)
49 ((oo)(oo))
56 (ooo(oo))
64 (oooooo)
86 (o(o(oo)))
106 (o(oooo))
196 (oo(oo)(oo))
224 (ooooo(oo))
256 (oooooooo)
301 ((oo)(o(oo)))
344 (ooo(o(oo)))
371 ((oo)(oooo))
424 (ooo(oooo))
454 (o((oo)(oo)))
526 (o(ooo(oo)))
622 (o(oooooo))
686 (o(oo)(oo)(oo))
784 (oooo(oo)(oo))
886 (o(o(o(oo))))
896 (ooooooo(oo))
MATHEMATICA
primeMS[n_]:=If[n===1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];
etQ[n_]:=Or[n===1, With[{m=primeMS[n]}, EvenQ@Length@m&&And@@etQ/@m]];
Select[Range[10000], etQ]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jan 13 2018
STATUS
approved