OFFSET
1,2
COMMENTS
934 of the first 1000 terms begin and end with the same digit. 40 of the first 1000 terms end in zero. Thus, only 26 of the first 1000 terms begin and end with different nonzero digits, with 153 being the smallest and 8026021 being the largest of those terms. - Harvey P. Dale, Jan 09 2021
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
EXAMPLE
820 and 028 = 28 both are triangular numbers hence both are members.
MATHEMATICA
Do[t = IntegerDigits[n(n + 1)/2]; u = t; u[[1]] = t[[ -1]]; u[[ -1]] = t[[1]]; t = FromDigits[u]; u = Floor[ Sqrt[2t]]; If[ u(u + 1)/2 == t, Print[n(n + 1)/2]], {n, 1, 300}]
sfl[n_]:=Module[{idn=IntegerDigits[n]}, FromDigits[Flatten[Join[{Last[ idn], Rest[ Most[ idn]], First[ idn]}]]]]; Join[ {1, 3, 6}, Select[ Accumulate[ Range[200]], OddQ[Sqrt[8 sfl[#]+1]]&]//Quiet] (* Harvey P. Dale, Jan 09 2021 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Amarnath Murthy, Apr 08 2002
EXTENSIONS
Edited, corrected and extended by Robert G. Wilson v
Edited by N. J. A. Sloane, Jan 20 2009
STATUS
approved