A077370 - OEIS

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

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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