MathOverflow will be down for maintenance for approximately 3 hours, starting Monday evening (06/24/2013) at approximately 9:00 PM Eastern time (UTC-4).
show/hide this revision's text 2 formatting

Mazur's proof that knots do not have inverses under addition of knots:
If A+B=0, $A+B=0$, then A $$A = A + (B+A)+(B+A)+..=(A+B)+(A+B)+..=0.B+A)+(B+A)+\cdots=(A+B)+(A+B)+\cdots=0.$$
This is like the traditional joke proof that 1=0 $1=0$ with A=1, B=-1; $A=1$, $B=-1$; the difference is that the proof with knots is valid because the infinite sums of knots are meaningful: make the knots smaller and smaller.

show/hide this revision's text 1 [made Community Wiki]

Mazur's proof that knots do not have inverses under addition of knots:
If A+B=0, then A = A + (B+A)+(B+A)+..=(A+B)+(A+B)+..=0.
This is like the traditional joke proof that 1=0 with A=1, B=-1; the difference is that the proof with knots is valid because the infinite sums of knots are meaningful: make the knots smaller and smaller.