We have two functions:

$f_1: \mathbb{N}\rightarrow \mathbb{N} \quad $ $f_2: \mathbb{N}\rightarrow \mathbb{N}$

By definition $f_1$ is turing-computable while $f_2$ is not.

Then we define a third funtion $g(n) = f_1(n) + f_2(n)$.

I want to show that $g(n)$ is not turing-computable:

So first assume that $g(n)$ is computable.

That means I can write it like this:

$f_2(n) = g(n)-f_1(n)$

Which (is where I'm not sure) means that $f_2$ can be computed which is a contradiction to the definition, which results that $f_2$ is not turing-computable.

  • $\begingroup$ The idea is correct, but the last equation is wrong: try to substitute $g(n)$ and note a simple error, which is easy to fix. Also, please read the description of the tag "check-my-answer" that you used. $\endgroup$
    – chi
    Nov 4, 2017 at 14:05
  • $\begingroup$ thank you, I fixed it, is there any way one could poke a hole in this logic ? $\endgroup$
    – zython
    Nov 4, 2017 at 14:10
  • 2
    $\begingroup$ Instead of saying "by definition $f_1$ is Turing-computable" you should say "suppose that $f_1$ is not Turing-computable" or "it is given that $f_1$ is not Turing-computable". There is no definition to speak of. $\endgroup$ Nov 4, 2017 at 14:50

1 Answer 1


This proof is correct as written, well done :) Remember this pattern, because an identical proof can be used in a wide variety of similar problems such as rational/irrational numbers and polynomials/non-polynomials


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