@tomleslie Nice. Vote up.

The weirdness surrounding 10 even applies when it's replaced by an abstract constant with assumptions. Compare:

limit((x-a)^n*x/x^(n+1), x= infinity) assuming n::posint, a>10;
limit((x-a)^n*x/x^(n+1), x= infinity) assuming n::posint, a<10;

@Kitonum You provide no clue as to how you constructed your function. Surely you did computations to get that.

@tomleslie While it's true that LinearAlgebra offers many solvers optimized to the structure of the coefficient matrix, it doesn't provide a dedicated tridiagonal solver; which is a shame, because that's a very important and commonly occurring case. 

The solver that I wrote (link given in the Answer below) outperforms by several times the best option provided by LinearAlgebra.

@acer No worries, I enjoy the opportunity to guess correctly. :-)

@acer Does WFH = "Work from home"?

@acer Thanks for the comparisons. The randomness is not session dependent. The key, which can be set with randomize(key), is always reset to 1 by a restart.

Very strange indeed. The cutoff at precisely 10 remains as you stated regardless of whether one uses decimals, exact fractions, or even exact transcendental constants! Try 11*sin(1), 12*sin(1), exp(2), exp(3), 3*Pi, 4*Pi.

I'm using Maple 2019.2.

Here is a much better algorithm for your task. It's better because rather than checking all numbers less than m for the presence of 2s in their base-3 representation, it generates only those numbers without 2s. It does this by converting numbers to base 2 and using their digits as the base-3 digits. 

rfin:= (m::nonnegint)->
    local M:= 1+floor(log[3](m+1)), E:= 3^~[$0..M], e, q; 
    remove(`>`, [seq(add(e*irem(b,2,'b'), e= E), b= 0..2^M)], m)
:     
rfin(50);
  [0, 1, 3, 4, 9, 10, 12, 13, 27, 28, 30, 31, 36, 37, 39, 40]

Again, to use this syntax, you must use the 1D input (aka Maple Input) mode (where, by default, the code appears dark red and monospaced, as above).

@acer The abbreviation of EmpiricalDistribution to Empirical that you've used seems to be undocumented. I don't have any problem with that; I just thought that it should be pointed out. I think that it's ridiculuous and unfortunate that so many names in Maple have redundant words tacked onto their ends. The GraphTheory package is one of the worse offenders.

Please don't repeat Questions from other threads. I read your other Question (the one I just deleted) in it's original thread yesterday, and I don't know the answer.

@emendes Change both instances of degree(p) to degree(p, var). The var at the end of the that line of code does nothing, and can be removed.

@dharr Thanks. I just noticed that myself, and now it's corrected.

@dharr I know that they don't appear in the specific example given, but surely 3*x should be considered linear and y^2 should be considered nonlinear.

@Mariusz Iwaniuk But notice the superscripts on the HypergeometricUs in the final series. They indicate derivatives that Mathematica can't expand. So, that answer is essentially the same as my series(' 'KummerU' '(p, 1/2, t), p= 0, 3).