OFFSET
1,1
COMMENTS
The sequence is infinite: the 10^n-th and the (10^n + 1)-th triangular numbers are members. It is a subsequence of A068898.
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
MAPLE
N:= 10: # to get all terms of up to 2N digits
Res:= NULL:
for n from 1 to N do
Divs:= select(t -> igcd(t, (10^n+1)/t)=1, numtheory:-divisors(10^n+1));
for d in Divs do
for e in [1, 3] do
u:= chrem([1, -1, e], [d, (10^n+1)/d, 4]);
y:= (u^2-1)/8/(10^n+1);
if y >= 10^(n-1) and y < 10^n then Res:= Res, y*(10^n+1) fi;
od od od:
sort([Res]); # Robert Israel, Feb 27 2017
MATHEMATICA
Select[Accumulate[Range[5*10^6]], EvenQ[IntegerLength[#]]&&Take[ IntegerDigits[ #], IntegerLength[ #]/2]== Take[IntegerDigits[#], -IntegerLength[#]/2]&] (* Harvey P. Dale, Aug 20 2022 *)
CROSSREFS
KEYWORD
AUTHOR
Amarnath Murthy, Mar 21 2002
EXTENSIONS
Corrected and extended by Larry Reeves (larryr(AT)acm.org), Jan 10 2003
STATUS
approved