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A143203
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Numbers having exactly two distinct prime factors p, q with q=p+4.
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5
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21, 63, 77, 147, 189, 221, 437, 441, 539, 567, 847, 1029, 1323, 1517, 1701, 2021, 2873, 3087, 3757, 3773, 3969, 4757, 5103, 5929, 6557, 7203, 8303, 9261, 9317, 9797, 10051, 11021, 11907, 12317, 15309, 16637, 21609
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| A143201(a(n)) = 5;
A020639(a(n))in A023200 and A006530(a(n)) in A046132;
subsequence of A007774: A001221(a(n))=2.
A033850 is a subsequence; subsequence of A195106. [Reinhard Zumkeller, Sep 13 2011]
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LINKS
| Reinhard Zumkeller, Table of n, a(n) for n = 1..250
Eric Weisstein's World of Mathematics, Cousin Primes
Index entries for primes, gaps between
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EXAMPLE
| a(1) = 21 = 3 * 7 = A023200(1) * A046132(1);
a(2) = 63 = 3^2 * 7 = A023200(1)^2 * A046132(1);
a(3) = 77 = 7 * 11 = A023200(2) * A046132(2);
a(4) = 147 = 3 * 7^2 = A023200(1) * A046132(1)^2;
a(5) = 189 = 3*3 * 7 = A023200(1)^3 * A046132(1);
a(6) = 221 = 13 * 17 = A023200(3) * A046132(3);
a(7) = 437 = 19 * 23 = A023200(4) * A046132(4);
a(8) = 441 = 3^2 * 7^2 = A023200(1)^2 * A046132(1)^2;
a(9) = 539 = 7^2 * 11 = A023200(2)^2 * A046132(2);
a(10) = 567 = 3^4 * 7 = A023200(1)^4 * A046132(1).
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PROG
| (Haskell)
a143203 n = a143203_list !! (n-1)
a143203_list = filter f [1, 3..] where
f x = length pfs == 2 && last pfs - head pfs == 4 where
pfs = a027748_row x
-- Reinhard Zumkeller, Sep 13 2011
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CROSSREFS
| Cf. A027748, A001221, A020639, A006530.
Sequence in context: A081302 A058100 A195106 * A082060 A025525 A033850
Adjacent sequences: A143200 A143201 A143202 * A143204 A143205 A143206
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KEYWORD
| nonn
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AUTHOR
| Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Aug 12 2008
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