

A072668


Numbers one less than composite numbers.


13



3, 5, 7, 8, 9, 11, 13, 14, 15, 17, 19, 20, 21, 23, 24, 25, 26, 27, 29, 31, 32, 33, 34, 35, 37, 38, 39, 41, 43, 44, 45, 47, 48, 49, 50, 51, 53, 54, 55, 56, 57, 59, 61, 62, 63, 64, 65, 67, 68, 69, 71, 73, 74, 75, 76, 77, 79, 80, 81, 83, 84, 85, 86, 87, 89, 90, 91, 92, 93, 94, 95
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OFFSET

1,1


COMMENTS

Complement of A006093 (primes minus 1); numbers which can be written as i*j+i+j, 0<i<=j: A072670(a(n))>0 for n>1; a(n)! is divisible by a(n)*(a(n)+1)/2, see A060462.
A072668 U A006093 = A000027. [JuriStepan Gerasimov, Oct 22 2009]


LINKS

Table of n, a(n) for n=1..71.


FORMULA

a(n) = A002808(n)  1.
a(n) = 2*A002808(n)  A079696(n). [JuriStepan Gerasimov, Oct 22 2009]


PROG

(PARI) for(n=2, 100, if(!isprime(n), print1(n1, ", "))) \\ Derek Orr, Jun 08 2015
(MAGMA) [n1: n in [2..120]  not IsPrime(n)]; // Vincenzo Librandi, Jun 09 2015


CROSSREFS

Cf. A066938, A072672.
Cf. A000027, A006093, A079696. [From JuriStepan Gerasimov, Oct 22 2009]
Sequence in context: A144724 A196990 A060462 * A249433 A300737 A062958
Adjacent sequences: A072665 A072666 A072667 * A072669 A072670 A072671


KEYWORD

nonn,easy


AUTHOR

Henry Bottomley, Apr 11 2001


STATUS

approved



