OFFSET
1,3
COMMENTS
The only known terms with an even number 2*m of digits that are the concatenation of two palindromes with m digits are 55, 66 and 828828 (see David Wells entry 828828). - Bernard Schott, Apr 29 2022
REFERENCES
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
Charles W. Trigg, Palindromic Triangular Numbers, J. Rec. Math., 6 (1973), 146-147.
David Wells, The Penguin Dictionary of Curious and Interesting Numbers, p. 73 and p. 178, entry 828828 (Rev. ed. 1997)
LINKS
T. D. Noe, Table of n, a(n) for n = 1..148 (from Patrick De Geest)
Patrick De Geest, Palindromic Triangulars
MATHEMATICA
palQ[n_]:=Module[{idn=IntegerDigits[n]}, idn==Reverse[idn]]; Select[ Accumulate[ Range[200000]], palQ] (* Harvey P. Dale, Mar 23 2011 *)
Select[Accumulate[Range[0, 170000]], PalindromeQ] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Sep 15 2019 *)
PROG
(PARI) list(lim)=my(v=List(), d); for(n=0, (sqrt(8*lim+1)-1)/2, d=digits(n*(n+1)/2); if(d==Vecrev(d), listput(v, n*(n+1)/2))); Vec(v) \\ Charles R Greathouse IV, Jun 23 2017
(Python)
A003098_list = [m for m in (n*(n+1)//2 for n in range(10**5)) if str(m) == str(m)[::-1]] # Chai Wah Wu, Sep 03 2021
CROSSREFS
KEYWORD
nonn,base,easy,nice
AUTHOR
STATUS
approved