Tribonacci

[raoulb]

Implementation of the Tribonacci constant via radicals.

[fredrik-johansson]

I think ca_tribonacci_constant should just create an element of the simple number field generated by qqbar_tribonacci_constant.

Other than that it looks good.

[raoulb]

I considered that, too. But then I modeled Tc in analogy to the golden ratio.
With my latest commit, it should be fine now.

Instead of the old output:

Evaluating Tc as ca:
1.83929 {(a+b+1)/3 where a = 3.30906 [Pow(36.2337 {3*c+19}, 0.333333 {1/3})], b = 1.20880 [Pow(1.76631 {-3*c+19}, 0.333333 {1/3})], c = 5.74456 [c^2-33=0]}

the example code now gives:

Evaluating Tc as ca:
1.83929 {a where a = 1.83929 [a^3-a^2-a-1=0]}

which at least looks much more useful indeed.

[raoulb]

In case of the golden ratio, the expression in terms of radicals is much simpler than here and hence the difference between both representations probably not such a big deal. What do you think, should we represent it as element of Q[a] where a^2 - a - 1 = 0?

I probably do not see through the details and consequences to really judge.
My main argument might just be for the sake of consistency.

BTW: I could try to do the changes if you agree.