OFFSET
1,2
COMMENTS
A prime index of n is a number m such that prime(m) divides n. A positive integer n is a Matula-Goebel number of a powerful rooted tree iff either n = 1 or n is a prime number whose prime index is a Matula-Goebel number of a powerful rooted tree or n is a powerful number (meaning its prime multiplicities are all greater than 1) whose prime indices are all Matula-Goebel numbers of powerful rooted trees.
EXAMPLE
The sequence of all powerful rooted trees together with their Matula-Goebel numbers begins:
1: o
2: (o)
3: ((o))
4: (oo)
5: (((o)))
7: ((oo))
8: (ooo)
9: ((o)(o))
11: ((((o))))
16: (oooo)
17: (((oo)))
19: ((ooo))
23: (((o)(o)))
25: (((o))((o)))
27: ((o)(o)(o))
31: (((((o)))))
MATHEMATICA
powgoQ[n_]:=Or[n==1, If[PrimeQ[n], powgoQ[PrimePi[n]], And[Min@@FactorInteger[n][[All, 2]]>1, And@@powgoQ/@PrimePi/@FactorInteger[n][[All, 1]]]]];
Select[Range[1000], powgoQ]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 30 2018
STATUS
approved