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

Author

M. F. Hasler, Jan 29 2014

Keywords

• nonn

Comments

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

Crossrefs

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

Programs

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)))

Formula

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