A077154 Smallest k such that there are as many composite numbers less than n as there are between n and k.

2, 3, 4, 5, 7, 9, 10, 11, 15, 17, 19, 21, 22, 23, 26, 28, 29, 31, 33, 34, 36, 39, 40, 41, 45, 47, 50, 52, 53, 55, 56, 57, 59, 63, 65, 67, 69, 70, 73, 76, 77, 78, 79, 81, 83, 86, 87, 88, 91, 93, 95, 97, 99, 100, 103, 106, 109, 112, 113, 115, 116, 117, 119, 121, 123, 125, 126

Offset

1,1

Comments

For n>3, the sequence lists 1 plus every composite number A002808(m), except for those A002808(2m) where A002808(m)+1 is also composite. E.g. 13 = 12+1 is not present because 12 is A002808(6) and A002808(3)+1 = 8+1 = 9 is composite. - Franklin T. Adams-Watters, Oct 11 2006

Links

Diana Mecum, Table of n, a(n) for n = 1..1000

Example

a(1)=2 because between 1 and 2 there is the same number of composites (none) as below 1. - R. J. Mathar, Sep 08 2008
For n = 12, there are 5 composites less than 12 (4,6,8,9,10), the next 5 composites are 14,15,16,18,20, so a(12) = 21.

Crossrefs

Cf. A077152, A077153, A002808, A092996, A092997, A092998.

Sequence in context: A004050 A123538 A092999 * A077273 A032955 A320848

Adjacent sequences: A077151 A077152 A077153 * A077155 A077156 A077157

Keyword

nonn

Author

Amarnath Murthy, Oct 31 2002

Extensions

More terms from Franklin T. Adams-Watters, Oct 11 2006

Edited by N. J. A. Sloane, Sep 18 2008 at the suggestion of R. J. Mathar.

Status

approved