1³+2³+3³+4³+5³+6³+7³+8³+9³+10³+11³+12³+13³+14³+15³+16³+17³+18³+19³+20³+21³+22³+23³+24³+25³+26³+...

From a channel Lumen includes in full: Numberphile, produced by Brady Haran, films working mathematicians (Hannah Fry, James Grime, Tony Padilla and many more) explaining real results, problems and proofs from their own fields, with the working shown on brown paper.

A 14-minute video from Numberphile's catalogue; the title names its topic.

Video by Numberphile, played through YouTube's embedded player.