|
|
A030231
|
|
Numbers with an even number of distinct prime factors.
|
|
21
|
|
|
1, 6, 10, 12, 14, 15, 18, 20, 21, 22, 24, 26, 28, 33, 34, 35, 36, 38, 39, 40, 44, 45, 46, 48, 50, 51, 52, 54, 55, 56, 57, 58, 62, 63, 65, 68, 69, 72, 74, 75, 76, 77, 80, 82, 85, 86, 87, 88, 91, 92, 93, 94, 95, 96, 98, 99, 100, 104, 106, 108, 111, 112, 115, 116, 117, 118, 119
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,2
|
|
COMMENTS
|
|
|
LINKS
|
|
|
FORMULA
|
k such that Sum_{d|k} mu(d)*A000005(d) = (-1)^omega(k) = +1 where mu(d)=A008683(d), and omega(d)=A001221(d).
|
|
MATHEMATICA
|
Select[Range[200], EvenQ[PrimeNu[#]]&] (* Harvey P. Dale, Jun 22 2011 *)
|
|
PROG
|
(PARI) j=[]; for(n=1, 200, x=omega(n); if(Mod(x, 2)==0, j=concat(j, n))); j
(Haskell)
a030231 n = a030231_list !! (n-1)
a030231_list = filter (even . a001221) [1..]
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn,easy,nice
|
|
AUTHOR
|
|
|
EXTENSIONS
|
Corrected by Dan Pritikin (pritikd(AT)muohio.edu), May 29 2002
|
|
STATUS
|
approved
|
|
|
|