A078260 a(n) = denominator(N), where N = 0.246...(2n) is the concatenation of the first n even numbers after decimal point.

5, 25, 500, 2500, 100000, 25000000, 5000000000, 125000000000, 50000000000000, 500000000000000, 500000000000000000, 6250000000000000000, 5000000000000000000000, 250000000000000000000000, 10000000000000000000000000, 1250000000000000000000000000, 500000000000000000000000000000

Offset

1,1

Links

Charlie Neder, Table of n, a(n) for n = 1..99

Maple

a:= n-> (t-> denom(t/10^length(t)))(parse(cat(2*i$i=1..n))):
seq(a(n), n=1..17);  # Alois P. Heinz, Jun 25 2025

Prog

(PARI) a(n) = {my(s = ""); for (k=1, n, s = concat(s, Str(2*k))); denominator(eval(s)/10^(#s)); } \\ Michel Marcus, Jan 15 2019
(PARI) a(n) = c = concat(vector(n, i, Str(2*i))); denominator(eval(c) / 10^#Str(c)) \\ David A. Corneth, Jan 15 2019

Crossrefs

Cf. A078257 (similar, with concatenation of 1 to n), A078261 (numerators).

Sequence in context: A082026 A101392 A385973 * A007185 A175852 A030995

Adjacent sequences: A078257 A078258 A078259 * A078261 A078262 A078263

Keyword

base,frac,nonn,easy

Author

Amarnath Murthy, Nov 24 2002

Extensions

More terms from Sascha Kurz, Jan 04 2003

a(5) and a(10) corrected, a(15)-a(17) from Charlie Neder, Jan 14 2019

Status

approved