We investigate the reverse mathematical strength of Turing determinacy up to Σ₅⁰, which is itself not provable in second order arithmetic.
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-fm27-12-2015, author = {Antonio Montalb\'an and Richard A. Shore}, title = {The strength of Turing determinacy within second order arithmetic}, journal = {Fundamenta Mathematicae}, volume = {233}, year = {2016}, pages = {249-268}, zbl = {06545385}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-fm27-12-2015} }
Antonio Montalbán; Richard A. Shore. The strength of Turing determinacy within second order arithmetic. Fundamenta Mathematicae, Tome 233 (2016) pp. 249-268. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-fm27-12-2015/