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

Author

Giovanni Teofilatto, May 04 2005

Keywords

• easy
• nonn

Examples

			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.
		

Links

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

Crossrefs

Cf. A001222, A001751, A105700.

Programs

Mathematica

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

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

Extensions

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