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A143731
Characteristic function of numbers with at least two distinct prime factors (A024619).
11
0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 1, 1, 1, 1, 0, 1, 1, 1, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1
OFFSET
1,1
COMMENTS
a(n) = 1 iff n is divisible by at least two distinct primes; otherwise 0.
FORMULA
a(n) = abs(A079872(n)) = A057427(A079275(n)).
For n>1: a(n) = 1 - floor(1/A001221(n)). - Enrique Pérez Herrero, Aug 13 2012
MATHEMATICA
Table[Boole[PrimeNu[n]>1], {n, 1, 200}] (* Enrique Pérez Herrero, Aug 13 2012 *)
KEYWORD
easy,nonn
AUTHOR
Reikku Kulon, Aug 30 2008
STATUS
approved