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%I #7 Dec 05 2013 19:55:52
%S 6,91,666,6786,48205,400065,2943951,56121715,497086215,5490000505,
%T 50127436765,401241809205,5699935500265,48641411367615,
%U 947211006213361,4036467268544805,35196287672814036,222222222111111111
%N Largest n-digit triangular number whose external digits as well as internal digits form a triangular number, or 0 if no such number exists.
%C At least one such number exists for every even n:
%C (2*10^(n/2)+1)*(10^(n/2)-1)/9, which gives lower bounds
%C a(2)>=21, a(4)>=2211, a(6)>=222111, a(8)>=22221111, etc., i.e., the triangular numbers (A000217) with indices 6, 66, 666, 6666, etc.
%Y Cf. A000217, A077366, A077367, A077368, A077369.
%K base,nonn
%O 1,1
%A _Amarnath Murthy_, Nov 06 2002
%E Offset corrected, terms a(2) and a(4) corrected, and terms a(6)-a(18) added by _Jon E. Schoenfield_, Jul 03 2010
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