# Total functional computable real numbers

Is there any computable real number which can not be computed by a higher order primitive recursive algorithm?

For computable real number I mean those that can be computed by a Turing machine to any desired precision in finite time. For higher order primitive recursive algorithm I mean common primitive recursive functions theory extended with first-class functions (as in Ackermann function).

Turing machines are more powerful than higher order primitive recursive functions so there exists the possibility that some computable reals numbers are not expressible by them.

• Could you include your preferred definition of "computable real number" and "higher order primitive recursive algorithm"? That will probably make answers more relevant (and also help answerers get some context.) – 6005 May 18 '17 at 21:55
• Interesting, I've never heard of "higher-order primitive recursion" before. Do you have a link to an authoritative definition? – gardenhead May 18 '17 at 22:46
• @gardenhead Not really. Wikipedia's Ackermann function entry mentions category theory before introducing it. I know it due to Total Functional Programming literature. – user3368561 May 18 '17 at 22:59

The set of higher-order primitive recursive reals is essentially the class of functions $\mathbb{N}\rightarrow\mathbb{N}$ which can be represented by a term $\mathrm{Nat}\rightarrow\mathrm{Nat}$ in Gödel's system T.