OFFSET
1,1
COMMENTS
Conjecture: the number of k > 0 such that a(k) <= n is asymptotic to C*log(n), C > 0.
Also, solutions x to the quadratic modular equation: x^2 + x + c ==0 (mod 10^a) where c = ((8-18*a)*10^a + 10^(2*a))/40, such that ceiling(10^(a-1)/2) <= x <= (10^a/2)-1, a=1,2,... - Herman Jamke (hermanjamke(AT)fastmail.fm), May 06 2007
EXAMPLE
0.2 + 0.4 + 0.6 + 0.8 = 2, hence 4 is in the sequence.
MATHEMATICA
s = 0; Do[s += (2*n)/10^Length[IntegerDigits[2*n]]; If[IntegerQ[s], Print[n]], {n, 1, 10^6}] (* Ryan Propper, Jul 30 2005 *)
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Benoit Cloitre, Aug 30 2003
EXTENSIONS
More terms from Ryan Propper, Jul 30 2005
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), May 06 2007
STATUS
approved