OFFSET
1,2
COMMENTS
A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798. The multiset multisystem with MM-number n is formed by taking the multiset of prime indices of each part of the multiset of prime indices of n. For example, the prime indices of 78 are {1,2,6}, so the multiset multisystem with MM-number 78 is {{},{1},{1,2}}. This sequence lists all MM-numbers of 2-regular (all vertex-degrees are 2) multiset partitions (no empty parts).
EXAMPLE
The sequence of multiset partitions whose MM-numbers belong to the sequence begins:
1: {}
7: {{1,1}}
9: {{1},{1}}
23: {{2,2}}
25: {{2},{2}}
97: {{3,3}}
121: {{3},{3}}
151: {{1,1,2,2}}
161: {{1,1},{2,2}}
169: {{1,2},{1,2}}
175: {{2},{2},{1,1}}
183: {{1},{1,2,2}}
185: {{2},{1,1,2}}
195: {{1},{2},{1,2}}
207: {{1},{1},{2,2}}
225: {{1},{1},{2},{2}}
227: {{4,4}}
289: {{4},{4}}
541: {{1,1,3,3}}
661: {{5,5}}
679: {{1,1},{3,3}}
687: {{1},{1,3,3}}
781: {{3},{1,1,3}}
841: {{1,3},{1,3}}
847: {{1,1},{3},{3}}
873: {{1},{1},{3,3}}
957: {{1},{3},{1,3}}
961: {{5},{5}}
MATHEMATICA
primeMS[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];
Select[Range[1, 100, 2], Or[#==1, SameQ[##, 2]&@@Last/@FactorInteger[Times@@primeMS[#]]]&]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Dec 17 2018
STATUS
approved