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 A227218 Smallest triangular number ending in n ones. 3
 1, 1711, 105111, 6271111, 664611111, 222222111111, 22222221111111, 2222222211111111, 2517912111111111, 18428299161111111111, 2222222222211111111111, 222222222222111111111111, 22222222222221111111111111, 1090161504430911111111111111 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS If a triangular number ends in like digits then it can only end in 0's, 1's, 5's or 6's. The sequence is infinite because the sequence of triangular numbers 21, 2211, 222111, 22221111, ... (A319170) is infinite. LINKS Giovanni Resta, Table of n, a(n) for n = 1..200 EXAMPLE a(2) = 1711 because 1711 is the smallest triangular number ending in  2 '1's. MATHEMATICA t = {}; Do[Do[x = n*(n + 1)/2; If[Mod[x, 10^m] == (10^m - 1)/9, AppendTo[t, x]; Break[]], {n, 1, 10^m}], {m, 1, 10}]; t a[n_] := Block[{x, y, s}, s = y /. List@ ToRules[ Reduce[(y+1)* y/2 == x*10^n +(10^n - 1)/9 && y > 0 && x >= 0, {y, x}, Integers] /. C[1] -> 0]; Min[s*(s + 1)/2]]; Array[a, 20] (* Giovanni Resta, Sep 20 2013 *) CROSSREFS Cf. A000217, A187127, A227219, A227220, A319170. Sequence in context: A241554 A129540 A293480 * A280113 A242102 A221481 Adjacent sequences:  A227215 A227216 A227217 * A227219 A227220 A227221 KEYWORD nonn,base AUTHOR Shyam Sunder Gupta, Sep 19 2013 EXTENSIONS a(11)-a(14) from Giovanni Resta, Sep 20 2013 STATUS approved

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Last modified May 26 00:02 EDT 2020. Contains 334612 sequences. (Running on oeis4.)