OFFSET
1,2
EXAMPLE
The sequence of anti-binary rooted trees together with their Matula-Goebel numbers begins:
1: o
2: (o)
3: ((o))
5: (((o)))
8: (ooo)
11: ((((o))))
12: (oo(o))
16: (oooo)
18: (o(o)(o))
19: ((ooo))
20: (oo((o)))
24: (ooo(o))
27: ((o)(o)(o))
30: (o(o)((o)))
31: (((((o)))))
MATHEMATICA
abQ[n_]:=Or[n==1, And[PrimeOmega[n]!=2, And@@Cases[FactorInteger[n], {p_, _}:>abQ[PrimePi[p]]]]]
Select[Range[100], abQ]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 15 2018
STATUS
approved