1,3,7,13,21,31 what come next? It supposed to be some thing simple, but some how I don't get it. 
I have a function with constant rate increase of 2. How can I calculate the n'th number?
It starts with 1, and it looks like this:
$$1,3,7,13,21,31,43...$$

First observe that the second differences are constant. 
Then we can assume it is $$f(x) = ax^2 + bx + c.$$
Subbing in $x = 0,1,2$ we have the system of equations
$$\pmatrix{1 & 0 & 0 & 1 \\ 1 & 1 & 1 & 3 \\ 1 & 2 & 4 & 7} = \pmatrix{1 & 0 & 0 & 1 \\ 0 & 1 & 1 & 2 \\ 0 & 2 & 4 & 6} = \pmatrix{1 & 0 & 0 & 1 \\ 0 & 1 & 1 & 2 \\ 0 & 0 & 2 & 2} = \pmatrix{1 & 0 & 0 & 1 \\ 0 & 1 & 0 & 1 \\ 0 & 0 & 1 & 1}.$$
Thus, we have $f(x) = x^2 + x + 1$.
For your interests.
If
$$a = 1 + 2 + \ldots + n, $$
then it is also true that 
$$2a = (1 + 2 + \ldots + n) + (n + (n - 1) + \ldots + 1) = (n + 1) + \ldots (n + 1) = n(n +1).$$
Thus, 

$$ 1 + 2 + \ldots + n = \frac{n(n+1)}{2}.$$

Finally, if $T(0) = 1$, then your sequence is just
$$T(n) = T(0) + 2 + 4 + \ldots + 2n = 1 + 2(1 + 2 + \ldots + n) = 1 + 2\frac{n(n+1)}{2} = n^2 + n + 1.$$
