# Before the Ides of March

If I lost a leg I'd be twice the size
When grown up I cause many multiplies
Maybe mistaken for a fifth in Rome
Or for twelve thirteenths in a modern home

What am I?

• Happy (British) Pie Week. Mar 14 '17 at 14:27

You are

$\pi$

If I lost a leg I'd be twice the size

By removing one vertical line from $\pi$ we get $\tau$, which is a standard mathematical notation for $2\pi$; see for example the links http://www.thepimanifesto.com/ and https://xkcd.com/1292/

When grown up I cause many multiplies

$\prod$ is used as symbol for products in mathematical equations.

Maybe mistaken for a fifth in Rome

Aicient Greek (not sure what this has to do with Rome) notation for 5 was something that resembles as PI. . Thanks to Jan Ivan

Or for twelve thirteenths in a modern home

I got nothing here. Still looking for something that matches.

Bonus (Before the Ides of March)

Happy Pi day. March 14. The Ides of March was March 15th in Rome.

• rome 5 is proly just = V, but greek ancient 5 = Π Mar 14 '17 at 9:56
• Line 2: In mathematics, the capital-pi is used to denote products (in the same way as the capital-sigma is used to denote summations) Mar 14 '17 at 12:03
• Two years ago I solved another puzzle in this spirit: puzzling.stackexchange.com/questions/10264/the-green-searcher Mar 14 '17 at 12:10
• @Gamow. I see your move, and answer with this move: tauday.com/tau-digits Mar 14 '17 at 13:21
• @Gamow, Also, from here: en.wikipedia.org/wiki/Pi "In 1958 Albert Eagle proposed replacing π by τ (tau), where τ = π/2, to simplify formulas. However, no other authors are known to use τ in this way. Some people use a different value, τ = 6.283185... = 2π ...." Mar 14 '17 at 13:25