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A077370
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Largest n-digit triangular number whose external digits as well as internal digits form a triangular number, or 0 if no such number exists.
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0
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6, 91, 666, 6786, 48205, 400065, 2943951, 56121715, 497086215, 5490000505, 50127436765, 401241809205, 5699935500265, 48641411367615, 947211006213361, 4036467268544805, 35196287672814036, 222222222111111111
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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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.
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CROSSREFS
| Cf. A000217, A077366, A077367, A077368, A077369.
Sequence in context: A177290 A000680 A013297 * A095864 A006151 A005327
Adjacent sequences: A077367 A077368 A077369 * A077371 A077372 A077373
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KEYWORD
| base,nonn
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AUTHOR
| Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Nov 06 2002
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EXTENSIONS
| Offset corrected, terms a(2) and a(4) corrected, and terms a(6)-a(18) added by Jon E. Schoenfield (jonscho(AT)hiwaay.net), Jul 03 2010
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