OFFSET
1,2
COMMENTS
Here a loop is an edge with two equal vertices, distinguished from a half-loop, which has only one vertex.
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 of multisets 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 of multisets with MM-number 78 is {{},{1},{1,2}}.
Also products of distinct primes whose prime indices are semiprimes, where a semiprime (A001358) is a product of any two prime numbers.
EXAMPLE
The sequence of terms together with their corresponding multisets of multisets (edge sets) begins:
1: {} 161: {{1,1},{2,2}} 347: {{2,9}}
7: {{1,1}} 163: {{1,8}} 373: {{1,12}}
13: {{1,2}} 167: {{2,6}} 377: {{1,2},{1,3}}
23: {{2,2}} 199: {{1,9}} 389: {{4,5}}
29: {{1,3}} 203: {{1,1},{1,3}} 421: {{1,13}}
43: {{1,4}} 227: {{4,4}} 439: {{3,7}}
47: {{2,3}} 233: {{2,7}} 443: {{1,14}}
73: {{2,4}} 257: {{3,5}} 449: {{2,10}}
79: {{1,5}} 269: {{2,8}} 467: {{4,6}}
91: {{1,1},{1,2}} 271: {{1,10}} 487: {{2,11}}
97: {{3,3}} 293: {{1,11}} 491: {{1,15}}
101: {{1,6}} 299: {{1,2},{2,2}} 499: {{3,8}}
137: {{2,5}} 301: {{1,1},{1,4}} 511: {{1,1},{2,4}}
139: {{1,7}} 313: {{3,6}} 553: {{1,1},{1,5}}
149: {{3,4}} 329: {{1,1},{2,3}} 559: {{1,2},{1,4}}
MATHEMATICA
Select[Range[100], SquareFreeQ[#]&&FreeQ[If[#==1, {}, FactorInteger[#]], {p_, k_}/; PrimeOmega[PrimePi[p]]!=2]&]
CROSSREFS
The case with only one edge is A106349.
The case covering an initial interval is A320461.
The version allowing multiple edges is A339112.
The half-loop version covering an initial interval is A340018.
The half-loop version is A340019.
A006450 lists primes of prime index.
A302242 is the weight of the multiset of multisets with MM-number n.
A309356 lists MM-numbers of simple graphs.
A339113 lists MM-numbers of multigraphs.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jan 02 2021
STATUS
approved