Znám's problem

In number theory, Znám's problem asks which sets of integers have the property that each integer in the set is a proper divisor of the product of the other integers in the set, plus 1.

Znám's problem is named after the Slovak mathematician Štefan Znám, who suggested it in 1972, although other mathematicians had considered similar problems around the same time.

The initial terms of Sylvester's sequence almost solve this problem, except that the last chosen term equals one plus the product of the others, rather than being a proper divisor.

Sun (1983) showed that there is at least one solution to the (proper) Znám problem for each

Sun's solution is based on a recurrence similar to that for Sylvester's sequence, but with a different set of initial values.

The Znám problem is closely related to Egyptian fractions.

It is unknown whether there are any solutions to Znám's problem using only odd numbers, and there remain several other open questions.

A closely related problem concerns sets of integers in which each integer in the set is a divisor, but not necessarily a proper divisor, of one plus the product of the other integers in the set.

Any solution to Znám's problem is also a solution to the improper Znám problem, but not necessarily vice versa.

Barbeau (1971) had posed the improper Znám problem for

, and Mordell (1973), independently of Znám, found all solutions to the improper problem for

Skula (1975) showed that Znám's problem is unsolvable for

, and credited J. Janák with finding the solution

that almost meets the conditions of Znám's problem, except that the largest value equals one plus the product of the other terms, rather than being a proper divisor.

[2] Thus, it is a solution to the improper Znám problem, but not a solution to Znám's problem as it is usually defined.

One solution to the proper Znám problem, for

A few calculations will show that Any solution to the improper Znám problem is equivalent (via division by the product of the values

Several of the cited papers on Znám's problem study also the solutions to this equation.

Brenton & Hill (1988) describe an application of the equation in topology, to the classification of singularities on surfaces,[2] and Domaratzki et al. (2005) describe an application to the theory of nondeterministic finite automata.

[3] The number of solutions to Znám's problem for any

is finite, so it makes sense to count the total number of solutions for each

[4] Sun (1983) showed that there is at least one solution to the (proper) Znám problem for each

Sun's solution is based on a recurrence similar to that for Sylvester's sequence, but with a different set of initial values.

[5] The number of solutions for small values of

, forms the sequence[6] Presently, a few solutions are known for

, but it is unclear how many solutions remain undiscovered for those values of

is not fixed: Cao & Jing (1998) showed that there are at least 39 solutions for each

, improving earlier results proving the existence of fewer solutions;[7] Sun & Cao (1988) conjecture that the number of solutions for each value of

[8] It is unknown whether there are any solutions to Znám's problem using only odd numbers.

If all numbers in a solution to Znám's problem or the improper Znám problem are prime, their product is a primary pseudoperfect number;[9] it is unknown whether infinitely many solutions of this type exist.