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A143203 Numbers having exactly two distinct prime factors p, q with q=p+4. 5
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; text; internal format)
OFFSET

1,1

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]

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

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).

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

CROSSREFS

Cf. A027748, A001221, A020639, A006530.

Sequence in context: A058100 A219856 A195106 * A082060 A025525 A033850

Adjacent sequences:  A143200 A143201 A143202 * A143204 A143205 A143206

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, Aug 12 2008

STATUS

approved

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Last modified August 24 04:58 EDT 2017. Contains 291052 sequences.