Splitting numbers, sum, repeat until single digits

[Cubidoo]

I am trying to achieve the following. I have a field called Number. In this field there can be any positive integer. If the integer consists of 2 or more digits then I need to split them and sum them. This I need to repeat until I have a single digit number.

For example:

Number = 37

Result should be 3 + 7 = 11 -> 1 + 1 = 2 (final result)

or

Number 134

Result 1 + 3 + 4= 8 (final result)

Which functions can I use to split numbers?

Which function can I use to repeat this process until I have a single digit?

[comment]

See if this helps: Link

[Cubidoo]

Thank you that link gave the solution