Must have been a temporary glitch? Worked for me this morning, still works…

Note also - a few years back, I started a thread about doing maths in the shell :

Anyway - I’d only just started learning how to use awk to do floating point arithmetic (in preference to using “bc”) - and when I saw this new thread, I went looking for some older shell scripts I might have written using awk to do floats, and couldn’t find a single example!

About the ((...)) notation, it should be noted that:

We can omit $ for variables in any arithmetic expression.
For example, we can write res=$((var1 + var2)) instead of res=$(($var1 + $var2))
As this rules applies for any arithmetic expression, not only within ((...)). For example, the following are valid:

i=3; str="abcdefgh"; echo ${str:i:i+1} (will be evaluated as echo ${str:3:4})

declare -i i=3; i=i+2; i+=5 (i value is 10)

the assignments operators (=, +=, <<=, ^= etc…) are valid in ((...)) construction. We can write ((res = var1 + var2)) instead res=$((var1 + var2))

((...)) can be nested. ((i = 2 + ((j = 4 * 10)))) will give i=42 and j=40.

I think you should give the full list of arithmetic operators, including comma, assignment, comparison, logical, bitwise and expr?expr:expr. They allow to have complex arithmetic expressions, such as ((i=3, j=4, k=i>j? i:j)) (k will be the max of i and j).

If you add the first 3, maybe you could also consider to include <</>> (and <<=/>>=) operators, as they are actually a multiplication/division by a power of 2: (( i <<= p )) is (( i *= ( 2 ** p ) )) and (( i >>= p )) is (( i /= ( 2 ** p ) )).
And also add an important warning on overflow, when we try to calculate over LONG_MAX and under LONG_MIN (defined in /usr/include/limits.h).
For example, on my system, LONG_MAX is 9223372036854775807, and we have an overflow with echo $((9223372036854775807 + 1)). bc (or my favorite dc) would be preferable in these cases, when numbers may go high/low.