|
|
A338901
|
|
Position of the first appearance of prime(n) as a factor in the list of squarefree semiprimes.
|
|
21
|
|
|
1, 1, 2, 3, 6, 7, 9, 11, 13, 17, 18, 21, 23, 25, 29, 31, 34, 36, 40, 42, 45, 47, 50, 52, 56, 58, 61, 64, 67, 70, 76, 78, 81, 82, 86, 89, 93, 97, 100, 104, 106, 107, 112, 113, 116, 118, 125, 129, 133, 134, 135, 139, 141, 147, 150, 154, 159, 160, 165, 167, 169
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,3
|
|
COMMENTS
|
The a(n)-th squarefree semiprime is the first divisible by prime(n).
After a(1) = 1, these are the positions of even terms in the list of all squarefree semiprimes A006881.
|
|
LINKS
|
|
|
FORMULA
|
|
|
MATHEMATICA
|
rs=First/@FactorInteger[#]&/@Select[Range[100], SquareFreeQ[#]&&PrimeOmega[#]==2&];
Table[Position[rs, i][[1, 1]], {i, Union@@rs}]
|
|
CROSSREFS
|
A004526 counts 2-part partitions, with strict case A140106 (shifted left).
A115392 is the not necessarily squarefree version.
A166237 gives the first differences of squarefree semiprimes.
A320656 counts factorizations into squarefree semiprimes.
A338899 gives prime indices of squarefree semiprimes, differences A338900.
Cf. A001221, A001222, A002100, A056239, A065516, A112798, A167171, A320891, A320911, A338903, A338905.
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|