A236675 If n is prime, then a(n) is the least composite number not occurring earlier, else a(n) is the least prime not occurring earlier.

2, 4, 6, 3, 8, 5, 9, 7, 11, 13, 10, 17, 12, 19, 23, 29, 14, 31, 15, 37, 41, 43, 16, 47, 53, 59, 61, 67, 18, 71, 20, 73, 79, 83, 89, 97, 21, 101, 103, 107, 22, 109, 24, 113, 127, 131, 25, 137, 139, 149, 151, 157, 26, 163, 167, 173, 179, 181, 27, 191, 28, 193

Offset

1,1

Comments

If "composite" is replaced by "nonprime", one gets A026234, a permutation of the integers.

Links

Table of n, a(n) for n=1..62.

Formula

a(n) = A002808(A000720(n)) if n is prime, a(n) = A000040(n-A000720(n)) else.

Prog

(PARI) A236675=n->if(isprime(n), A002808(primepi(n)), prime(n-primepi(n)))
(PARI) c=p=1; vector(99, n, if(isprime(n), while(isprime(c++), ); c, p=nextprime(p+1)))

Crossrefs

Cf. A026234, A002808, A018252, A097457, A236675.

Sequence in context: A358209 A057336 A340646 * A210771 A359535 A115316

Adjacent sequences: A236672 A236673 A236674 * A236676 A236677 A236678

Keyword

nonn

Author

M. F. Hasler, Jan 29 2014

Status

approved