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A008509 Positive integers k such that k-th triangular number is palindromic. 15
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 *)

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

Patrick De Geest

STATUS

approved

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Last modified April 3 22:48 EDT 2020. Contains 333207 sequences. (Running on oeis4.)