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A009188 Short leg of more than one Pythagorean triangle. 5
9, 12, 15, 16, 18, 20, 21, 24, 25, 27, 28, 30, 32, 33, 35, 36, 39, 40, 42, 44, 45, 48, 49, 50, 51, 52, 54, 55, 56, 57, 60, 63, 64, 65, 66, 68, 69, 70, 72, 75, 76, 77, 78, 80, 81, 84, 85, 87, 88, 90, 91, 92, 93, 95, 96, 98, 99, 100, 102, 104, 105, 108, 110, 111, 112, 114, 115, 116 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Values of n for which composite n X n magic squares are possible. - J. Lowell, May 20 2010

If n is in the sequence, k*n is in the sequence for all k > 1. So odd semiprimes (A046315) and numbers of the form 4*p where p is an odd prime are core subsequences which give the initial terms of arithmetic progressions in this sequence. - Altug Alkan, Nov 29 2015

Numbers appearing more than once in A009004. - Sean A. Irvine, Apr 20 2018

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

MAPLE

filter:= proc(n) not isprime(n) and (n::odd or not isprime(n/2)) end proc:

select(filter, [$9 .. 10000]); # Robert Israel, Nov 30 2015

MATHEMATICA

filterQ[n_] := !PrimeQ[n] && (OddQ[n] || !PrimeQ[n/2]);

Select[Range[9, 120], filterQ] (* Jean-François Alcover, Feb 28 2019, from Maple *)

PROG

(PARI) forcomposite(n=9, 1e3, if(n % 2 == 1 ­­|| !isprime(n/2), print1(n, ", "))) \\ Altug Alkan, Dec 01 2015

CROSSREFS

Cf. A001749, A009004, A020884, A046315, A264828.

Sequence in context: A170951 A044859 A114306 * A138299 A050435 A176656

Adjacent sequences:  A009185 A009186 A009187 * A009189 A009190 A009191

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified December 16 04:05 EST 2019. Contains 330013 sequences. (Running on oeis4.)