  We believe we have made progress in the age-old problem of divisibility rules
for integers. Universal divisibility rule is introduced for any divisor in any
base number system. The divisibility criterion is written down explicitly as a
linear form in the digits of the test number. The universal criterion contains
only two parameters that depend on the divisor and are easily calculated. These
divisibility parameters are not unique for a given divisor but each
two-parameter set yields a unique criterion. Well-known divisibility rules for
exemplary divisors in the decimal system follow from the universal expression
as special cases.
