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A105661 a(n)=1 if n is a prime, 2 if n is an even semiprime, otherwise 0. 3
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 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,4
LINKS
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
Sequence in context: A337518 A073781 A048622 * A082451 A121362 A234694
KEYWORD
easy,nonn
AUTHOR
Giovanni Teofilatto, May 04 2005
EXTENSIONS
Corrected and extended by Rick L. Shepherd, May 17 2005
STATUS
approved

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Last modified April 20 12:25 EDT 2024. Contains 371844 sequences. (Running on oeis4.)