%I #11 Nov 21 2013 12:50:19
%S 21,63,77,105,147,189,221,315,437,441,525,539,567,735,847,945,1029,
%T 1323,1517,1575,1701,2021,2205,2625,2835,2873,3087,3675,3757,3773,
%U 3969,4725,4757,5103,5145,5929,6557,6615,7203,7875,8303,8505,9261,9317,9797,10051
%N Numbers with largest and smallest prime factors differing by 4.
%H Reinhard Zumkeller, <a href="/A195106/b195106.txt">Table of n, a(n) for n = 1..250</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/CousinPrimes.html">Cousin Primes</a>
%e a(10) = 441 = 3^2 * 7^2;
%e a(11) = 525 = 3 * 5^2 * 7;
%e a(12) = 539 = 7^2 * 11.
%t pf4Q[n_]:=Module[{pfs=Transpose[FactorInteger[n]][[1]]}, Max[pfs]- Min[pfs]==4]; Select[Range[11000],pf4Q] (* _Harvey P. Dale_, Sep 24 2011 *)
%o (Haskell)
%o a195106 n = a195106_list !! (n-1)
%o a195106_list = filter (\x -> a006530 x - a020639 x == 4) [1,3..]
%Y Cf. A195118, A143206; A143203 is a subsequence.
%K nonn
%O 1,1
%A _Reinhard Zumkeller_, Sep 13 2011