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 A225172 #26 Jun 13 2025 07:39:53
%S A225172 1,2,6,14,42,86,186,462,930,1986,4170,6510,14682,29366,50342,73410,
%T A225172 189498,287654,491190,849570,1699142,2433878,4280774,7978218,14442690,
%U A225172 25900142,44400390,78492954,123958794,228018066,355388970,629582370,780686294
%N A225172 Largest number which requires n iterations of the unitary totient function (A047994) to reach 1.
%C A225172 Searched up to 10^10. a(33) >= 1609056138. a(34) >= 2275537110. a(35) >= 4171607222. - _Donovan Johnson_, May 02 2013
%H A225172 M. Lal, <a href="http://dx.doi.org/10.1090/S0025-5718-1974-0335419-3">Iterates of the unitary totient function</a>, Math. Comp., 28 (1974), 301-302.
%F A225172 a(n) = max{x : A049865(x) = n}. - _R. J. Mathar_, May 02 2013
%t A225172 (* This is just a verification of recorded data up to a(23), assuming a(n-1)/2 <= a(n) <= 4*a(n-1) *) uphi[n_] := (cnt++; fi = FactorInteger[n]; Times @@ (fi[[All, 1]]^fi[[All, 2]] - 1)); f[n_] := (cnt = 0; NestWhile[uphi, n, # > 1 &]; cnt); a[0] = 1; a[1] = 2; a[n_] := a[n] = (For[record = k = a[n-1]/2//Floor, k <= 4*a[n-1], k++, If[f[k] == n, record = k]]; record); Table[Print[a[n]]; a[n], {n, 0, 23}] (* _Jean-François Alcover_, May 02 2013 *)
%Y A225172 Cf. A047994, A003271, A225173.
%K A225172 nonn
%O A225172 0,2
%A A225172 _N. J. A. Sloane_, May 01 2013
%E A225172 a(16)-a(32) from _Donovan Johnson_, May 02 2013