OFFSET
1,1
COMMENTS
A squarefree semiprime is a product of any two distinct prime numbers.
Do all terms belong to A242031 (weakly decreasing prime signature)?
EXAMPLE
The sequence of terms together with their prime indices begins:
6: {1,2}
60: {1,1,2,3}
840: {1,1,1,2,3,4}
12600: {1,1,1,2,2,3,3,4}
264600: {1,1,1,2,2,2,3,3,4,4}
5821200: {1,1,1,1,2,2,2,3,3,4,4,5}
151351200: {1,1,1,1,1,2,2,2,3,3,4,4,5,6}
The sequence of terms together with their prime signatures begins:
6: (1,1)
60: (2,1,1)
840: (3,1,1,1)
12600: (3,2,2,1)
264600: (3,3,2,2)
5821200: (4,3,2,2,1)
151351200: (5,3,2,2,1,1)
4994589600: (5,4,2,2,2,1)
169816046400: (6,4,2,2,2,1,1)
5943561624000: (6,4,3,3,2,1,1)
225855341712000: (7,4,3,3,2,1,1,1)
8808358326768000: (7,5,3,3,2,2,1,1)
405184483031328000: (8,5,3,3,2,2,1,1,1)
MATHEMATICA
FoldList[Times, Select[Range[20], SquareFreeQ[#]&&PrimeOmega[#]==2&]]
CROSSREFS
A166237 gives first differences of squarefree semiprimes.
A320655 counts factorizations into semiprimes.
A320656 counts factorizations into squarefree semiprimes.
A338901 gives first appearances in the list of squarefree semiprimes.
A339113 gives products of primes of squarefree semiprime index.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Nov 30 2020
STATUS
approved