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A067270
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Numbers n such that n-th triangular number (A000217) ends in n.
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0
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1, 5, 25, 625, 9376, 90625, 890625, 7109376, 12890625, 212890625, 1787109376, 81787109376, 59918212890625, 259918212890625, 3740081787109376, 56259918212890625, 256259918212890625, 7743740081787109376
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| T_n = n(n+1)/2. Thanks to David W. Wilson for the proof that this sequence is a proper subset of A003226.
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EXAMPLE
| T_5 = 15 ends in 5, hence 5 is a term of the sequence.
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MATHEMATICA
| a5=A018247 less the commas; a6=A018248 less the commas; b5 = FromDigits[ Reverse[ IntegerDigits[a5]]]; b6 = FromDigits[ Reverse[ IntegerDigits[a6]]]; f[0] = 1; f[n_] := Block[{c5 = Mod[b5, 10^n], c6 = Mod[b6, 10^n]}, If[ Mod[c5(c5 + 1)/2, 10^n] == c5, c5, c6]]; Union[ Table[ f[n], {n, 0, 20}]]
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CROSSREFS
| Proper subset of A003226. Cf. A007185, A018247, A016090, A018248.
Sequence in context: A007185 A175852 A030995 * A176594 A169652 A137114
Adjacent sequences: A067267 A067268 A067269 * A067271 A067272 A067273
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KEYWORD
| base,nonn
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AUTHOR
| Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Feb 21 2002
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EXTENSIONS
| Edited and extended by Robert G. Wilson v (rgwv(AT)rgwv.com), Nov 20 2002
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