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 A053795 Composite numbers ending in 1, 3, 7 or 9. 4
 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+9)*(10y+9), (10x+11)*(10y+11)}, {(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+9)*(10y+11)}, with x, y integers >= 0. - Marvin Y. Hubble, Jul 12 2013 and May 12 2014 and Sep 27 2019 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 (Python) from sympy import isprime def ok(n): return n > 1 and n%10 in {1, 3, 7, 9} and not isprime(n) print(list(filter(ok, range(2, 288)))) # Michael S. Branicky, Sep 21 2021 CROSSREFS Subsequence of A045572. Sequence in context: A338909 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 January 28 20:13 EST 2023. Contains 359905 sequences. (Running on oeis4.)