[Xavier]

The number sequence is determined by taking the sums of the square roots of the digits of the previous terms, hence the ?? after 145_ is 20

Edit: The full sequence is 2, 4, 16, 37, 58, 89, 145, 42, 20, 4, 16, 37, 58, 89, 145, 42, 20, 4 and loops indefinitely.

You mean you have to sum the squares of each digit to get the next number (not the square root)

Nicely done