from D. Suryanarayana (1972). [The number of bi-unitary divisors of an integer, in The Theory of Arithmetic Functions, Lecture Notes in Mathematics 251: 273–282, New York, Springer–Verlag]. The number of bi-unitary divisors of n is a multiplicative function of n with average order where A bi-unitary perfect number is one equal to the sum of its bi-unitary aliquot divisors. The only such numbers are 6, 60 and 90. OEIS sequences References Section B3. Section 4.2 External links Mathoverflow | Boolean ring of unitary divisors Number theory