A077370 Largest n-digit triangular number whose external digits as well as internal digits form a triangular number, or 0 if no such number exists.

6, 91, 666, 6786, 48205, 400065, 2943951, 56121715, 497086215, 5490000505, 50127436765, 401241809205, 5699935500265, 48641411367615, 947211006213361, 4036467268544805, 35196287672814036, 222222222111111111

Offset

1,1

Comments

At least one such number exists for every even n:

(2*10^(n/2)+1)*(10^(n/2)-1)/9, which gives lower bounds

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.

Links

Table of n, a(n) for n=1..18.

Crossrefs

Cf. A000217, A077366, A077367, A077368, A077369.

Sequence in context: A219304 A305324 A265088 * A226382 A095864 A246155

Adjacent sequences: A077367 A077368 A077369 * A077371 A077372 A077373

Keyword

base,nonn

Author

Amarnath Murthy, Nov 06 2002

Extensions

Offset corrected, terms a(2) and a(4) corrected, and terms a(6)-a(18) added by Jon E. Schoenfield, Jul 03 2010

Status

approved