A299322 Ranks of {2,3}-power towers with neither consecutive 2's nor consecutive 3's; see Comments.

1, 2, 4, 5, 10, 11, 22, 23, 45, 48, 92, 97, 185, 196, 372, 393, 745, 788, 1492, 1577, 2985, 3156, 5972, 6313, 11945, 12628, 23892, 25257, 47785, 50516, 95572, 101033, 191145, 202068, 382292, 404137, 764585, 808276, 1529172, 1616553, 3058345, 3233108, 6116692, 6466217

Offset: 1

Author

Clark Kimberling, Feb 07 2018

Keywords

• nonn

Comments

Suppose that S is a set of real numbers. An S-power-tower, t, is a number t = x(1)^x(2)^...^x(k), where k >= 1 and x(i) is in S for i = 1..k. We represent t by (x(1), x(2), ..., x(k)), which for k > 1 is defined as (x(1), (x(2), ..., x(k))); (2,3,2) means 2^9. The number k is the *height* of t. If every element of S exceeds 1 and all the power towers are ranked in increasing order, the position of each in the resulting sequence is its *rank*. See A299229 for a guide to related sequences.

Examples

			The first seven terms are the ranks of these towers: t(1) = (2), t(2) = (3), t(4) = (2,3), t(5) = (3,2), t(10) = (2,3,2), t(11) = (3,2,3), t(22) = (3,2,3,2).
		

Crossrefs

Cf. A299229.

Programs

Mathematica

t[1] = {2}; t[2] = {3}; t[3] = {2, 2}; t[4] = {2, 3}; t[5] = {3, 2};
t[6] = {2, 2, 2}; t[7] = {3, 3}; t[8] = {3, 2, 2}; t[9] = {2, 2, 3};
t[10] = {2, 3, 2}; t[11] = {3, 2, 3}; t[12] = {3, 3, 2};
z = 190; g[k_] := If[EvenQ[k], {2}, {3}]; f = 6;
While[f < 13, n = f; While[n < z, p = 1;
  While[p < 12, m = 2 n + 1; v = t[n]; k = 0;
    While[k < 2^p, t[m + k] = Join[g[k], t[n + Floor[k/2]]]; k = k + 1];
   p = p + 1; n = m]]; f = f + 1]
Select[Range[5000], Max[Map[Length, Split[t[#]]]] < 2 &]

Formula

Conjectures from Colin Barker, Feb 09 2018: (Start)

G.f.: x*(1 + x + x^2 + x^4 - 2*x^5 + 2*x^6 - 2*x^7 + x^8) / ((1 - x)*(1 + x^2)*(1 - 2*x^2)).

a(n) = a(n-1) + a(n-2) - a(n-3) + 2*a(n-4) - 2*a(n-5) for n >= 10.

(End)

Extensions

a(37)-a(44) from Pontus von Brömssen, Aug 08 2024