A105661 a(n)=1 if n is a prime, 2 if n is an even semiprime, otherwise 0.

0, 1, 1, 2, 1, 2, 1, 0, 0, 2, 1, 0, 1, 2, 0, 0, 1, 0, 1, 0, 0, 2, 1, 0, 0, 2, 0, 0, 1, 0, 1, 0, 0, 2, 0, 0, 1, 2, 0, 0, 1, 0, 1, 0, 0, 2, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 2, 1, 0, 1, 2, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 2, 0, 0, 0, 0, 1, 0, 0, 2, 1, 0, 0, 2, 0, 0, 1, 0, 0, 0, 0, 2, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0

Offset

1,4

Links

Antti Karttunen, Table of n, a(n) for n = 1..10000

Example

a(3) = 1 because n = 3 is a prime;
a(4) = 2 because n = 4 = 2*2 is an even semiprime;
a(8) = 0 because n = 8 is neither prime nor an even semiprime.
a(15) = 0 because n = 15 = 3*5 is an odd semiprime.

Mathematica

Table[Which[PrimeQ[n], 1, EvenQ[n]&&PrimeOmega[n]==2, 2, True, 0], {n, 120}] (* Harvey P. Dale, Jul 05 2021 *)

Prog

(PARI) for(n=1, 105, if((n%2==0)&&(bigomega(n)==2), r=2, r=isprime(n)); print1(r, ", "))
(Scheme) (define (A105661 n) (cond ((= 1 (A001222 n)) 1) ((and (even? n) (= 2 (A001222 n))) 2) (else 0))) ;; Antti Karttunen, Jul 26 2017

Crossrefs

Cf. A001222, A001751, A105700.

Sequence in context: A337518 A073781 A048622 * A082451 A121362 A234694

Adjacent sequences: A105658 A105659 A105660 * A105662 A105663 A105664

Keyword

easy,nonn

Author

Giovanni Teofilatto, May 04 2005

Extensions

Corrected and extended by Rick L. Shepherd, May 17 2005

Status

approved