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A053795 Composite numbers ending in 1, 3, 7 or 9. 3
9, 21, 27, 33, 39, 49, 51, 57, 63, 69, 77, 81, 87, 91, 93, 99, 111, 117, 119, 121, 123, 129, 133, 141, 143, 147, 153, 159, 161, 169, 171, 177, 183, 187, 189, 201, 203, 207, 209, 213, 217, 219, 221, 231, 237, 243, 247, 249, 253, 259, 261, 267, 273, 279, 287 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Composite numbers not divisible by 2 or 5. - Lekraj Beedassy, Jul 05 2004

Composite numbers ending in 1, 3, 7 or 9 are values (some shared within sets, because some values are numbers with multiple factors) of the following sets of binomial products:

  {(10x+3)*(10y+7), (10x+11)*(10y+11), (10x+19)*(10y+19)}, {(10x+3)*(10y+11), (10x+7)*(10y+9)},

  {(10x+3)*(10y+9), (10x+7)*(10y+11)}, and

  {(10x+3)*(10y+3), (10x+7)*(10y+7), (10x+11)*(10y+19)}, with x, y integers >= 0. - Marvin Y. Hubble, Jul 12 2013 and May 12 2014

LINKS

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

FORMULA

a(n) = 2.5n + 2.5n/log n + O(n/log^2 n). - Charles R Greathouse IV, Jan 30 2018

MAPLE

remove(isprime, [seq(seq(10*i+j, j=[3, 7, 9, 11]), i=0..100)]); # Robert Israel, Jan 29 2018

MATHEMATICA

Select[Range[300], CompositeQ[#]&&OddQ[#]&&!Divisible[#, 5]&] (* Harvey P. Dale, Jul 13 2014 *)

PROG

(PARI) is(n)=gcd(n, 10)==1 && !isprime(n) && n>1 \\ Charles R Greathouse IV, Jan 30 2018

CROSSREFS

Sequence in context: A273202 A173456 A192192 * A100490 A173250 A128462

Adjacent sequences:  A053792 A053793 A053794 * A053796 A053797 A053798

KEYWORD

nonn,base,easy

AUTHOR

G. L. Honaker, Jr., Apr 01 2000

EXTENSIONS

More terms from James A. Sellers, Apr 08 2000

Offset corrected by Arkadiusz Wesolowski, Dec 18 2011

STATUS

approved

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Last modified July 19 09:33 EDT 2019. Contains 325155 sequences. (Running on oeis4.)