This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A078221 #13 Jan 12 2022 10:38:53 %S A078221 1,3,9,99,9999,99999999,9999999999999999, %T A078221 99999999999999999999999999999999, %U A078221 9999999999999999999999999999999999999999999999999999999999999999 %N A078221 a(1) = 1, a(n+1) > a(n) is the smallest multiple of a(n) using only odd digits. %F A078221 a(n) = 10^(2^(n-3)) - 1 for n >= 3. (Proof by induction. Consider a(n)*f, L = ceiling(log(f)/log(10)), g1 = number formed by the first L digits of a(n)*f, g2 = number formed by the last L digits of a(n)*f => g1 + g2 = number formed by L 9's, if L <= 10^(2^(n-2)) + 1). - _Sascha Kurz_, Jan 04 2003 %p A078221 1,3,seq(10^(2^(n-3))-1,n=3..11); %o A078221 (Python) %o A078221 def A078221(n): return 2*n-1 if n < 3 else 10**(2**(n-3)) - 1 # _Chai Wah Wu_, Jan 12 2022 %Y A078221 Cf. A078222. %K A078221 base,nonn %O A078221 1,2 %A A078221 _Amarnath Murthy_, Nov 22 2002 %E A078221 More terms from _Sascha Kurz_, Jan 04 2003