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 A068899 Triangular numbers containing 2n digits obtained by duplicating the first n digits; i.e., triangular numbers in A020338. 5
 55, 66, 5050, 5151, 203203, 255255, 426426, 500500, 501501, 581581, 828828, 930930, 39653965, 50005000, 50015001, 61566156, 3347133471, 5000050000, 5000150001, 6983669836, 220028220028, 500000500000, 500001500001 (list; graph; refs; listen; history; text; internal format)
 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 CROSSREFS Cf. A020338, A068898. Sequence in context: A168109 A116055 A068898 * A053719 A217236 A050781 Adjacent sequences:  A068896 A068897 A068898 * A068900 A068901 A068902 KEYWORD easy,nonn,base,look AUTHOR Amarnath Murthy, Mar 21 2002 EXTENSIONS Corrected and extended by Larry Reeves (larryr(AT)acm.org), Jan 10 2003 STATUS approved

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Last modified August 16 10:10 EDT 2022. Contains 356161 sequences. (Running on oeis4.)