A rigorous elementary proof of the Basel problem [6, 1] ∑n=1∞1n2=π26 is formalized in the Mizar system [3]. This theorem is item 14 from the “Formalizing 100 Theorems” list maintained by Freek Wiedijk at http://www.cs.ru.nl/F.Wiedijk/100/.
@article{bwmeta1.element.doi-10_1515_forma-2017-0014, author = {Karol P\k ak and Artur Korni\l owicz}, title = {Basel Problem}, journal = {Formalized Mathematics}, volume = {25}, year = {2017}, pages = {149-155}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.doi-10_1515_forma-2017-0014} }
Karol Pąk; Artur Korniłowicz. Basel Problem. Formalized Mathematics, Tome 25 (2017) pp. 149-155. http://gdmltest.u-ga.fr/item/bwmeta1.element.doi-10_1515_forma-2017-0014/