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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 (list; graph; refs; listen; history; text; internal format)
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. [Juri-Stepan 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). [Juri-Stepan Gerasimov, Oct 22 2009]

PROG

(PARI) for(n=2, 100, if(!isprime(n), print1(n-1, ", "))) \\ Derek Orr, Jun 08 2015

(MAGMA) [n-1: n in [2..120] | not IsPrime(n)]; // Vincenzo Librandi, Jun 09 2015

CROSSREFS

Cf. A066938, A072672.

Cf. A000027, A006093, A079696. [From Juri-Stepan 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

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Last modified January 19 12:52 EST 2020. Contains 331049 sequences. (Running on oeis4.)