if in formulas
Iverson's notation Iverson bracket
https://gyazo.com/f34157fc1cb1fb44299121220725502a
[Iverson's notation - Wikipedia https://ja.wikipedia.org/wiki/%E3%82%A2%E3%82%A4%E3%83%90%E3%83%BC%E3%82%BD%E3%83%B3%E3%81%AE%E8%A8%98%E6%B3% 95]
$ \sum_{x:0<x<N} f(x) = \sum_x f(x)[0<x<N]
Personally, I didn't like Kronecker's delta and indicator functions because they were hard to read, but I thought this notation was nice because it didn't make it too hard to read.
When neither True nor False has a non-zero value, it is easier to read if it is written in pieces rather than put together (B below).
A: $ (x < 0 ? -1 : 1)
B: $ -1[x<0] + 1[x\ge0]
C: $ 1 -2[x<0]
D: $ -1\delta_{x<0} + 1 \delta_{x\ge0}
E: $ 1 - 2\delta_{x<0}
Two notes on notation - Donald E. Knuth
$ [k \,\,\mathrm{even}] [k \,\,\mathrm{odd}] [k \,\,\mathrm{prime}] [k \,\,\mathrm{divedes} \,\,n]
Kronecker delta
https://gyazo.com/b0463982ecb09c045acdd774d66c27d5
[Kronecker's Delta - Wikipedia https://ja.wikipedia.org/wiki/%E3%82%AF%E3%83%AD%E3%83%8D%E3%83%83%E3%82%AB%E3%83%BC%E3%81%AE%E3%83%87%E3%83% AB%E3%82%BF]
indicator function
https://gyazo.com/86ae63fcd61a851bd0318e7e2afc1615
Directive function - Wikipedia
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