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%I #2 Mar 30 2012 17:26:28
%S 337,737,7373,73333,73733,333377,333733,737333,737773,777337,777737,
%T 3333373,3333773,3373337,3373777,3777377,3777733,7337737,7373333,
%U 7373773,7733377,7733777,7777373,33337333,33337733,33377737,33733373
%N Numbers of form 21k+1 or 21k+2 that use only digits 3 and 7.
%Y Cf. A152824 and A152834.
%K base,nonn
%O 1,1
%A _Zak Seidov_, Dec 14 2008, Dec 26 2008
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