The set of all indices of all functions provably recursive in any reasonable theory $T$ is shown to be recursively isomorphic to $U\times\overline{U}$, where $U$ is $\Pi_2$-complete set.
@article{119119, author = {V\'\i t\v ezslav \v Svejdar}, title = {Arithmetical classification of the set of all provably recursive functions}, journal = {Commentationes Mathematicae Universitatis Carolinae}, volume = {40}, year = {1999}, pages = {631-634}, zbl = {1010.03049}, mrnumber = {1756541}, language = {en}, url = {http://dml.mathdoc.fr/item/119119} }
Švejdar, Vítězslav. Arithmetical classification of the set of all provably recursive functions. Commentationes Mathematicae Universitatis Carolinae, Tome 40 (1999) pp. 631-634. http://gdmltest.u-ga.fr/item/119119/
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