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 A008509 Positive integers k such that k-th triangular number is palindromic. 17
 1, 2, 3, 10, 11, 18, 34, 36, 77, 109, 132, 173, 363, 1111, 1287, 1593, 1833, 2662, 3185, 3369, 3548, 8382, 11088, 18906, 50281, 57166, 102849, 111111, 167053, 179158, 246642, 337650, 342270, 365436, 417972, 1620621, 3240425, 3457634, 3707883 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES Charles W. Trigg, Palindromic Triangular Numbers, J. Rec. Math., 6 (1973), 146-147. D. Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, 93. LINKS T. D. Noe, Table of n, a(n) for n = 1..147 (from P. De Geest) P. De Geest, Palindromic Triangulars MATHEMATICA palQ[n_]:= Reverse[x=IntegerDigits[n]]==x; t={}; Do[If[palQ[n*(n+1)/2], AppendTo[t, n]], {n, 4*10^6}]; t (* Jayanta Basu, May 13 2013 *) Position[Accumulate[Range[371*10^4]], _?PalindromeQ]//Flatten (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jun 12 2020 *) PROG (PARI) ispal(n)=n=digits(n); for(i=1, #n\2, if(n[i]!=n[#n+1-i], return(0))); 1 is(n)=ispal(n*(n+1)/2) \\ Charles R Greathouse IV, May 15 2013 (MAGMA) [k:k in [1..5000000]| Intseq(Binomial(k+1, 2)) eq Reverse(Intseq(Binomial(k+1, 2)))]; //  Marius A. Burtea, Jul 16 2019 CROSSREFS Cf. A003098, A002113, A000217, A083571, A050721, A050722. Sequence in context: A248407 A047473 A270474 * A281366 A324921 A307034 Adjacent sequences:  A008506 A008507 A008508 * A008510 A008511 A008512 KEYWORD nonn,base AUTHOR STATUS approved

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Last modified May 16 03:04 EDT 2022. Contains 353687 sequences. (Running on oeis4.)