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 A087473 #27 Feb 24 2024 01:06:31
%S A087473 1,10,25,39,77,171,199,577,887,1592,2682,3988,6913,18747,39577,58439,
%T A087473 99428,173442,267343,299137,574182,685812,880543,1635812,1974447,
%U A087473 2771717,18871813,45797337,49899368,58935768,158504329,265956179,566800111,896125563
%N A087473 Smallest positive number that requires n iterations of f(k) to reach a single digit, where f(k) is the product of the two numbers formed from the alternating digits of k.
%H A087473 Giovanni Resta, <a href="/A087473/b087473.txt">Table of n, a(n) for n = 0..40</a>
%e A087473 a(4)= 77 since 77 is the smallest number that requires 4 iterations to reach a single digit: f(77)=7*7=49, f(49)=4*9=36, f(36)=3*6=18, f(18)=1*8=8.
%t A087473 f[n_] := Block[{d = IntegerDigits@ n}, If[OddQ@ Length@ d, PrependTo[d, 0]]; Times @@ FromDigits /@ Transpose@ Partition[d, 2]]; a[n_] := Block[ {c=-1, m}, t=0; While[c != n, t++; m=t; c=0; While[m > 9, c++; m = f@ m]]; t]; a /@ Range[0, 12] (* _Giovanni Resta_, Aug 01 2018 *)
%Y A087473 Cf. A087471, A087472, A087474.
%K A087473 nonn,base
%O A087473 0,2
%A A087473 _Amarnath Murthy_ and _Paul D. Hanna_, Sep 11 2003
%E A087473 More terms from _Ray Chandler_, Sep 19 2003
%E A087473 a(30)-a(33) from _Giovanni Resta_, Aug 01 2018