A302049 a(n) = 1 if n = prime(k)*prime(1+k) for some k, otherwise 0.

0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0

Offset: 1

Author

Antti Karttunen, Apr 24 2018

Keywords

• nonn

Comments

Characteristic function for products of 2 successive primes (A006094).

Links

• Antti Karttunen, Table of n, a(n) for n = 1..67591
• Index entries for characteristic functions
• Index entries for sequences computed from indices in prime factorization

Crossrefs

Cf. A006094, A078898, A137794, A246277, A280710, A302047, A302048.

Programs

PARI

A302049(n) = if(n<=1,0,my(p=precprime(sqrtint(n))); p>1 && 0==(n%p) && isprime(n/p) && (nextprime(p+1)==n/p)); \\ After code in A006094

PARI

first(n) = my(res = vector(n), p = 2); forprime(q = 3, , if(p * q > n, return(res)); res[p * q]++; p = q) \\ David A. Corneth, Apr 24 2018

Formula

a(n) = A137794(n) * A280710(n).

a(n) = A185013(A246277(n)) = A185013(A078898(n)).