OFFSET
1,1
COMMENTS
Numbers n such that A109082(n) = 3.
EXAMPLE
The sequence of all rooted trees of depth 3 together with their Matula-Goebel numbers begins:
5: (((o)))
10: (o((o)))
13: ((o(o)))
15: ((o)((o)))
17: (((oo)))
20: (oo((o)))
23: (((o)(o)))
25: (((o))((o)))
26: (o(o(o)))
30: (o(o)((o)))
34: (o((oo)))
35: (((o))(oo))
37: ((oo(o)))
39: ((o)(o(o)))
40: (ooo((o)))
43: ((o(oo)))
45: ((o)(o)((o)))
46: (o((o)(o)))
50: (o((o))((o)))
51: ((o)((oo)))
52: (oo(o(o)))
60: (oo(o)((o)))
MATHEMATICA
Select[Range[100], Length[NestWhileList[Times@@PrimePi/@FactorInteger[#][[All, 1]]&, #, #>1&]]-1==3&]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Mar 21 2019
STATUS
approved