• Karyoplasma@discuss.tchncs.de
    3·
    1 year ago

    For divisibility by 13, take the last number, multiply by 4 and add to the rest.

    For divisibility by 17, take the last number, multiply by 5 and subtract from the rest.

    For divisibility by 19, take the last number, multiply by 2 and add to the rest.

    In fact, you can adapt the method to check for divisibility by any prime number k.