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A077154
Smallest k such that there are as many composite numbers less than n as there are between n and k.
5
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
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
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