I've always been fascinated by how linear programming seems to be applicable to every problem under the sun but SAT solvers only really seem to be good at Sudoku.
In practice that are a bunch of problems that seem to be SAT, but they are either SAT at a scale where the solver still can't find a solution in any reasonable time or they turn out to not really be SAT because there is that one extra constraint that is quite difficult to encode in simple logic.
And it is unrewarding experimenting because it ends up being a day or so remembering how to use a SAT solver, then rediscovering how horrible raw sat solver interfaces are and trying to find a library that builds SAT problems from anything other than raw boolean algebra (the intros really undersell how bad the experience of using SAT solvers directly is, the DIMACS sat file format makes me think of the year 1973), then discovering that a SAT solver can't actually solve the alleged-SAT problem. Although if anyone is ever looking for a Clojure library I can recommend rolling-stones [0] as having a pleasant API to work with.
I love SAT solvers, but way more underappreciated by software engineers are MILP solvers.
MILPs (Mixed Integer Linear Programs) are basically sets of linear constraints, along with a linear optimization functions, where your variables can either be reals or ints.
Notably, you can easily encode any SAT problem as a MILP, but it's much easier to encode optimization problems, or problems with "county" constraints as MILPs.
This is interesting, but techniques like CDCL seem to only ever find any one valuation that makes a proposition true. My homegrown solver finds all valuations that make a proposition true and then can eliminate redundancies, such that `X or not X and Y` gets simplified to `X or Y`, just to mention one example (proof: truth tables are identical). Are there any other SAT solvers out there that do something like that? My own one suffers from combinatorial explosion in the simplification stage when expressions get "complex" enough. But then, the simplification is NP-complete, AFAICT.
I don't understand why SAT solvers don't use gaussian elimination more. Every SAT problem can be represented as an intersection of linear (XORSAT) and 2SAT clauses, and the linear system can resolve some common contradictions, propagate literals, etc.
Also Grobner basis algorithm over polynomials in Z_2 can be used to solve SAT. A SAT problem can be encoded as a set of quadratic polynomials, and if the generated ideal is all polynomials, the system is unsatisfiable (that's Nullstellenansatz). I don't understand how we can get high degree polynomials when running Grobner basis algorithm that specifically prefers low degree polynomials. It intuitively doesn't make sense.
If you convert a sudoku to an exact cover you can usually solve it by finding colums that are a subset from another column and remove all rows that are only in one of them.
Sudokus that can be solved with reasoning alone, can be solved in polynomial time.
I recently discovered that solving exact covers, and probably also with SAT, using a generic strategy, does not always result in the most efficient way for finding solutions. There are problems that have 10^50 solutions, yer finding a solution can take a long time.
Problems, including NP-complete ones, are only a product of the way you look at them and the reference frame from where you look at them. They get their incarnation only out of the observer's context.
Slightly related, there’s ways to differentiate linear programs (https://github.com/cvxpy/cvxpylayers), which might allow one to endow deep neural networks with some similar reasoning capabilities as these sorts of solvers.
SATisfying Solutions to Difficult Problems
(vaibhavsagar.com)92 points by atilimcetin 22 October 2025 | 52 comments
Comments
In practice that are a bunch of problems that seem to be SAT, but they are either SAT at a scale where the solver still can't find a solution in any reasonable time or they turn out to not really be SAT because there is that one extra constraint that is quite difficult to encode in simple logic.
And it is unrewarding experimenting because it ends up being a day or so remembering how to use a SAT solver, then rediscovering how horrible raw sat solver interfaces are and trying to find a library that builds SAT problems from anything other than raw boolean algebra (the intros really undersell how bad the experience of using SAT solvers directly is, the DIMACS sat file format makes me think of the year 1973), then discovering that a SAT solver can't actually solve the alleged-SAT problem. Although if anyone is ever looking for a Clojure library I can recommend rolling-stones [0] as having a pleasant API to work with.
[0] https://github.com/Engelberg/rolling-stones
MILPs (Mixed Integer Linear Programs) are basically sets of linear constraints, along with a linear optimization functions, where your variables can either be reals or ints.
Notably, you can easily encode any SAT problem as a MILP, but it's much easier to encode optimization problems, or problems with "county" constraints as MILPs.
Also Grobner basis algorithm over polynomials in Z_2 can be used to solve SAT. A SAT problem can be encoded as a set of quadratic polynomials, and if the generated ideal is all polynomials, the system is unsatisfiable (that's Nullstellenansatz). I don't understand how we can get high degree polynomials when running Grobner basis algorithm that specifically prefers low degree polynomials. It intuitively doesn't make sense.
Sudokus that can be solved with reasoning alone, can be solved in polynomial time.
I recently discovered that solving exact covers, and probably also with SAT, using a generic strategy, does not always result in the most efficient way for finding solutions. There are problems that have 10^50 solutions, yer finding a solution can take a long time.
https://www.youtube.com/watch?v=_GP9OpZPUYc