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A143205 Numbers having exactly two distinct prime factors p, q with q=p+6. 4
55, 91, 187, 247, 275, 391, 605, 637, 667, 1147, 1183, 1375, 1591, 1927, 2057, 2491, 3025, 3127, 3179, 3211, 4087, 4459, 4693, 4891, 5767, 6647, 6655, 6875, 7387, 8281, 8993, 9991, 10807, 11227, 12091, 15125, 15341, 15379, 17947, 19343, 22627, 23707 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A143201(a(n)) = 7;

A020639(a(n))in A023201 and A006530(a(n)) in A046117;

subsequence of A007774: A001221(a(n))=2; A111192 is a subsequence.

Subsequence of A195118. [Reinhard Zumkeller, Sep 13 2011]

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..250

Eric Weisstein's World of Mathematics, Sexy Primes

Index entries for primes, gaps between

EXAMPLE

a(1) = 55 = 5 * 11 = A023201(1) * A046117(1);

a(2) = 91 = 7 * 13 = A023201(2) * A046117(2);

a(3) = 187 = 11 * 17 = A023201(3) * A046117(3);

a(4) = 247 = 13 * 19 = A023201(4) * A046117(4);

a(5) = 275 = 5^2 * 11 = A023201(1)^2 * A046117(1);

a(6) = 391 = 17 * 23 = A023201(5) * A046117(5);

a(7) = 605 = 5 * 11^2 = A023201(1) * A046117(1)^2;

a(8) = 637 = 7^2 * 13 = A023201(2)^2 * A046117(2);

a(9) = 667 = 23 * 29 = A023201(6) * A046117(6);

a(10) = 1147 = 31 * 37 = A023201(7) * A046117(7).

MATHEMATICA

okQ[n_]:=Module[{fi=Transpose[FactorInteger[n]][[1]]}, Length[fi]==2 && Last[fi]-First[fi]==6]; Select[Range[25000], okQ]  (* Harvey P. Dale, Apr 18 2011 *)

PROG

(Haskell)

a143205 n = a143205_list !! (n-1)

a143205_list = filter f [1, 3..] where

   f x = length pfs == 2 && last pfs - head pfs == 6 where

       pfs = a027748_row x

-- Reinhard Zumkeller, Sep 13 2011

CROSSREFS

Cf. A027748, A001221, A020639, A006530.

Sequence in context: A027865 A247681 A195118 * A111192 A063873 A063131

Adjacent sequences:  A143202 A143203 A143204 * A143206 A143207 A143208

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, Jul 30 2008

STATUS

approved

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Last modified November 20 00:29 EST 2019. Contains 329323 sequences. (Running on oeis4.)