OFFSET
1,2
COMMENTS
A squarefree semiprime is a product of any two distinct prime numbers. A prime index of n is a number m such that the m-th prime number divides n. The multiset of prime indices of n is row n of A112798.
Is this sequence an anti-run, i.e., are there no adjacent equal parts? I have verified this conjecture up to n = 10^6. - Gus Wiseman, Nov 18 2020
FORMULA
MATHEMATICA
-Subtract@@PrimePi/@First/@FactorInteger[#]&/@Select[Range[100], SquareFreeQ[#]&&PrimeOmega[#]==2&]
CROSSREFS
A176506 is the not necessarily squarefree version.
A338899 has row-differences equal to this sequence.
A338901 gives positions of first appearances.
A001221 counts distinct prime indices.
A001222 counts prime indices.
A001358 lists semiprimes.
A005117 lists squarefree numbers.
A065516 gives first differences of semiprimes.
A166237 gives first differences of squarefree semiprimes.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Nov 16 2020
STATUS
approved