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

Author

Amarnath Murthy, Nov 24 2002

Keywords

• base
• frac
• nonn
• easy

Links

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

Crossrefs

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

Programs

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

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

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