%I A078972
%S A078972 4,6,9,10,14,15,21,25,35,49,121,143,169,187,209,221,247,253,289,299,319,
%T A078972 323,341,361,377,391,403,407,437,451,473,481,493,517,527,529,533,551,
%U A078972 559,583,589,611,629,649,667,671,689,697,703,713,731,737,767,779,781
%N A078972 Brilliant numbers: semiprimes (products of two primes, A001358) whose 
               prime factors have the same number of decimal digits.
%C A078972 "Brilliant numbers, as defined by Peter Wallrodt, are numbers with two 
               prime factors of the same length (in decimal notation). These numbers 
               are generally used for cryptographic purposes and for testing the 
               performance of prime factoring programs." [Alpern]
%D A078972 P. D. James, The Private Patient, Knopf, NY, 2008, p. 192. (from N. J. 
               A. Sloane, Aug 27 2009)
%H A078972 T. D. Noe, <a href="b078972.txt">Table of n, a(n) for n=1..10537</a>
%H A078972 Dario Alpern, <a href="http://www.alpertron.com.ar/BRILLIANT.HTM">Brilliant 
               Numbers</a>.
%e A078972 1711 = 29*59 is in the sequence since both of its factors have two digits.
%t A078972 fQ[n_] := Block[{fi = FactorInteger@n}, Plus @@ Last /@ fi == 2 && Floor[ 
               Log[10, fi[[1, 1]] ]] == Floor[ Log[10, fi[[ -1, 1]] ]]]; Select[ 
               Range@792, fQ@# &] (from Robert G. Wilson v (rgwv(AT)rgwv.com), May 
               26 2006)
%Y A078972 Cf. A001358, A085647.
%Y A078972 Sequence in context: A113433 A115654 A036326 this_sequence A115652 A084759 
               A054395
%Y A078972 Adjacent sequences: A078969 A078970 A078971 this_sequence A078973 A078974 
               A078975
%K A078972 base,easy,nonn
%O A078972 1,1
%A A078972 Jason Earls (zevi_35711(AT)yahoo.com), Jan 12 2003
%E A078972 Edited by N. J. A. Sloane, Aug 27 2009