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a(n) = denominator(N), where N = 0.246...up to 2n (concatenation 2 to 2n of first n even numbers after decimal point).
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%I #19 May 06 2022 13:13:51

%S 5,25,500,2500,100000,25000000,5000000000,125000000000,50000000000000,

%T 500000000000000,500000000000000000,6250000000000000000,

%U 5000000000000000000000,250000000000000000000000,10000000000000000000000000,1250000000000000000000000000,500000000000000000000000000000

%N a(n) = denominator(N), where N = 0.246...up to 2n (concatenation 2 to 2n of first n even numbers after decimal point).

%H Charlie Neder, <a href="/A078260/b078260.txt">Table of n, a(n) for n = 1..99</a>

%o (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

%o (PARI) a(n) = c = concat(vector(n, i, Str(2*i))); denominator(eval(c) / 10^#Str(c)) \\ _David A. Corneth_, Jan 15 2019

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

%K base,frac,nonn,easy

%O 1,1

%A _Amarnath Murthy_, Nov 24 2002

%E More terms from _Sascha Kurz_, Jan 04 2003

%E a(5) and a(10) corrected, a(15)-a(17) from _Charlie Neder_, Jan 14 2019