Of all the ideas we’ve forgotten from math class, the most important to relearn – because it illuminates our most urgent challenges – is the difference between arithmetic and geometric progressions.

Of all the ideas we’ve forgotten from math class, the most important to relearn — because it illuminates our most urgent challenges — is the difference between arithmetic and geometric progressions.

A long-standing puzzle seems to constrain how addition and multiplication relate to each other. A graduate student has gone further than anyone else in establishing the connection. In 1983 the ...