create for documenting POSIX shell’s undefined $((x ** y)) for exponents

SC3019.md

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+## In POSIX sh, exponentials are undefined.
+
+### Problematic code:
+
+ShellCheck noticed you're using `**`-notation with two asterisks to obtain an exponent's value. Some examples:
+
+```sh
+#!/bin/sh
+echo $((2 ** 3)) # Using `**` for an exponent is undefined in POSIX sh.
+```
+
+### Correct code:
+
+Other possibilities exist:
+```sh
+#!/bin/sh
+echo $((2 * 2 * 2))   # equivalent to `bash`'s `echo $((2 ** 3))`
+echo '2 ^ 3' | bc     # piping the formula to `bc` to parse the output https://stackoverflow.com/a/13111995
+printf '2 ^ 3\n' | bc # piping using `printf` (newline `\n` is mandatory)
+```
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX!