feat: add QuadraticDiophantineEquations model (#543) (#812)

* feat: add QuadraticDiophantineEquations model (#543)

Add decision problem: given positive integers a, b, c, determine whether
there exist positive integers x, y such that ax^2 + by = c. Single
variable x with y derived; complexity O(sqrt(c)) by trial.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix agentic review issues: rename c_val→c getter, fix paper solve command

- Rename `c_val()` getter to `c()` so user-facing catalog output shows
  the mathematical parameter name instead of an implementation detail
- Add `--solver brute-force` to paper's `pred solve` command since
  QuadraticDiophantineEquations has no ILP reduction path

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: rename duplicate rhs CLI field to qde_rhs for QuadraticDiophantineEquations

* fix: remove OLA→STMWCT rule file accidentally included from merge

The file was brought in by a contaminated merge commit and does not
belong to the QuadraticDiophantineEquations model PR.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: rename --qde-rhs to --coeff-c for consistency

All three QDE parameters now use the coeff- prefix: --coeff-a, --coeff-b, --coeff-c.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: zazabap <sweynan@icloud.com>

Author: 3 people

docs/paper/reductions.typ

@@ -178,6 +178,7 @@
   "PrecedenceConstrainedScheduling": [Precedence Constrained Scheduling],
   "PrimeAttributeName": [Prime Attribute Name],
   "QuadraticAssignment": [Quadratic Assignment],
+  "QuadraticDiophantineEquations": [Quadratic Diophantine Equations],
   "QuantifiedBooleanFormulas": [Quantified Boolean Formulas (QBF)],
   "RectilinearPictureCompression": [Rectilinear Picture Compression],
   "ResourceConstrainedScheduling": [Resource Constrained Scheduling],
@@ -3257,6 +3258,53 @@ A classical NP-complete problem from Garey and Johnson @garey1979[Ch.~3, p.~76],
   ]
 }
 
+#{
+  let x = load-model-example("QuadraticDiophantineEquations")
+  let a = x.instance.a
+  let b = x.instance.b
+  let c = x.instance.c
+  let config = x.optimal_config
+  let xval = config.at(0) + 1
+  let yval = int((c - a * xval * xval) / b)
+  // Enumerate all valid x values for the table
+  let max-x = calc.floor(calc.sqrt(c / a))
+  let rows = range(1, max-x + 1).map(xi => {
+    let rem = c - a * xi * xi
+    let feasible = rem > 0 and calc.rem(rem, b) == 0
+    let yi = if feasible { int(rem / b) } else { none }
+    (xi, rem, feasible, yi)
+  })
+  [
+    #problem-def("QuadraticDiophantineEquations")[
+      Given positive integers $a$, $b$, $c$, determine whether there exist positive integers $x$, $y$ such that $a x^2 + b y = c$.
+    ][
+      Quadratic Diophantine equations of the form $a x^2 + b y = c$ form one of the simplest families of mixed-degree Diophantine problems. The variable $y$ is entirely determined by $x$ via $y = (c - a x^2) slash b$, so the decision problem reduces to checking whether any $x in {1, dots, floor(sqrt(c slash a))}$ yields a positive integer $y$. This can be done in $O(sqrt(c))$ time by trial#footnote[No algorithm improving on brute-force trial of all candidate $x$ values is known; the registered complexity `sqrt(c)` reflects this direct enumeration bound.].
+
+      *Example.* Let $a = #a$, $b = #b$, $c = #c$. Then $x$ ranges over $1, dots, #max-x$:
+
+      #pred-commands(
+        "pred create --example QuadraticDiophantineEquations -o qde.json",
+        "pred solve qde.json --solver brute-force",
+        "pred evaluate qde.json --config " + config.map(str).join(","),
+      )
+
+      #align(center, table(
+        columns: 4,
+        align: center,
+        table.header([$x$], [$c - a x^2$], [Divisible by $b$?], [$y$]),
+        ..rows.map(((xi, rem, ok, yi)) => (
+          [$#xi$],
+          [$#rem$],
+          [#if ok [Yes] else [No]],
+          [#if yi != none [$#yi$] else [$dash$]],
+        )).flatten(),
+      ))
+
+      The instance is satisfiable: $x = #xval, y = #yval$ gives $#a dot #xval^2 + #b dot #yval = #c$.
+    ]
+  ]
+}
+
 #{
   let x = load-model-example("ClosestVectorProblem")
   let basis = x.instance.basis

problemreductions-cli/src/cli.rs

@@ -249,6 +249,7 @@ Flags by problem type:
   ProductionPlanning             --num-periods, --demands, --capacities, --setup-costs, --production-costs, --inventory-costs, --cost-bound
   SubsetSum                       --sizes, --target
   ThreePartition                  --sizes, --bound
+  QuadraticDiophantineEquations    --coeff-a, --coeff-b, --coeff-c
   SumOfSquaresPartition           --sizes, --num-groups
   ExpectedRetrievalCost           --probabilities, --num-sectors
   PaintShop                       --sequence
@@ -754,6 +755,15 @@ pub struct CreateArgs {
     /// Target string for StringToStringCorrection (comma-separated symbol indices, e.g., "0,1,3,2")
     #[arg(long)]
     pub target_string: Option<String>,
+    /// Coefficient a for QuadraticDiophantineEquations (coefficient of x²)
+    #[arg(long)]
+    pub coeff_a: Option<u64>,
+    /// Coefficient b for QuadraticDiophantineEquations (coefficient of y)
+    #[arg(long)]
+    pub coeff_b: Option<u64>,
+    /// Constant c for QuadraticDiophantineEquations (right-hand side of ax² + by = c)
+    #[arg(long)]
+    pub coeff_c: Option<u64>,
 }
 
 #[derive(clap::Args)]

problemreductions-cli/src/commands/create.rs

@@ -9,7 +9,8 @@ use anyhow::{bail, Context, Result};
 use problemreductions::export::{ModelExample, ProblemRef, ProblemSide, RuleExample};
 use problemreductions::models::algebraic::{
     ClosestVectorProblem, ConsecutiveBlockMinimization, ConsecutiveOnesMatrixAugmentation,
-    ConsecutiveOnesSubmatrix, FeasibleBasisExtension, SparseMatrixCompression, BMF,
+    ConsecutiveOnesSubmatrix, FeasibleBasisExtension, QuadraticDiophantineEquations,
+    SparseMatrixCompression, BMF,
 };
 use problemreductions::models::formula::Quantifier;
 use problemreductions::models::graph::{
@@ -193,7 +194,10 @@ fn all_data_flags_empty(args: &CreateArgs) -> bool {
         && args.conjuncts_spec.is_none()
         && args.deps.is_none()
         && args.query.is_none()
+        && args.coeff_a.is_none()
+        && args.coeff_b.is_none()
         && args.rhs.is_none()
+        && args.coeff_c.is_none()
         && args.required_columns.is_none()
 }
 
@@ -728,6 +732,7 @@ fn example_for(canonical: &str, graph_type: Option<&str>) -> &'static str {
         "IntegerKnapsack" => "--sizes 3,4,5,2,7 --values 4,5,7,3,9 --capacity 15",
         "SubsetSum" => "--sizes 3,7,1,8,2,4 --target 11",
         "ThreePartition" => "--sizes 4,5,6,4,6,5 --bound 15",
+        "QuadraticDiophantineEquations" => "--coeff-a 3 --coeff-b 5 --coeff-c 53",
         "BoyceCoddNormalFormViolation" => {
             "--n 6 --sets \"0,1:2;2:3;3,4:5\" --target 0,1,2,3,4,5"
         }
@@ -2418,6 +2423,32 @@ pub fn create(args: &CreateArgs, out: &OutputConfig) -> Result<()> {
             )
         }
 
+        // QuadraticDiophantineEquations
+        "QuadraticDiophantineEquations" => {
+            let a = args.coeff_a.ok_or_else(|| {
+                anyhow::anyhow!(
+                    "QuadraticDiophantineEquations requires --coeff-a, --coeff-b, and --coeff-c\n\n\
+                     Usage: pred create QuadraticDiophantineEquations --coeff-a 3 --coeff-b 5 --coeff-c 53"
+                )
+            })?;
+            let b = args.coeff_b.ok_or_else(|| {
+                anyhow::anyhow!(
+                    "QuadraticDiophantineEquations requires --coeff-b\n\n\
+                     Usage: pred create QuadraticDiophantineEquations --coeff-a 3 --coeff-b 5 --coeff-c 53"
+                )
+            })?;
+            let c = args.coeff_c.ok_or_else(|| {
+                anyhow::anyhow!(
+                    "QuadraticDiophantineEquations requires --coeff-c\n\n\
+                     Usage: pred create QuadraticDiophantineEquations --coeff-a 3 --coeff-b 5 --coeff-c 53"
+                )
+            })?;
+            (
+                ser(QuadraticDiophantineEquations::try_new(a, b, c).map_err(anyhow::Error::msg)?)?,
+                resolved_variant.clone(),
+            )
+        }
+
         // SumOfSquaresPartition
         "SumOfSquaresPartition" => {
             let sizes_str = args.sizes.as_deref().ok_or_else(|| {
@@ -7703,7 +7734,10 @@ mod tests {
             storage: None,
             quantifiers: None,
             homologous_pairs: None,
+            coeff_a: None,
+            coeff_b: None,
             rhs: None,
+            coeff_c: None,
             required_columns: None,
         }
     }

src/models/algebraic/mod.rs

@@ -8,6 +8,7 @@
 //! - [`ConsecutiveBlockMinimization`]: Consecutive Block Minimization
 //! - [`ConsecutiveOnesSubmatrix`]: Consecutive Ones Submatrix (column selection with C1P)
 //! - [`QuadraticAssignment`]: Quadratic Assignment Problem
+//! - [`QuadraticDiophantineEquations`]: Decide ax² + by = c in positive integers
 //! - [`SparseMatrixCompression`]: Sparse Matrix Compression by row overlay
 
 pub(crate) mod bmf;
@@ -18,6 +19,7 @@ pub(crate) mod consecutive_ones_submatrix;
 pub(crate) mod feasible_basis_extension;
 pub(crate) mod ilp;
 pub(crate) mod quadratic_assignment;
+pub(crate) mod quadratic_diophantine_equations;
 pub(crate) mod qubo;
 pub(crate) mod sparse_matrix_compression;
 
@@ -29,6 +31,7 @@ pub use consecutive_ones_submatrix::ConsecutiveOnesSubmatrix;
 pub use feasible_basis_extension::FeasibleBasisExtension;
 pub use ilp::{Comparison, LinearConstraint, ObjectiveSense, VariableDomain, ILP};
 pub use quadratic_assignment::QuadraticAssignment;
+pub use quadratic_diophantine_equations::QuadraticDiophantineEquations;
 pub use qubo::QUBO;
 pub use sparse_matrix_compression::SparseMatrixCompression;
 
@@ -44,6 +47,7 @@ pub(crate) fn canonical_model_example_specs() -> Vec<crate::example_db::specs::M
     specs.extend(consecutive_ones_submatrix::canonical_model_example_specs());
     specs.extend(feasible_basis_extension::canonical_model_example_specs());
     specs.extend(quadratic_assignment::canonical_model_example_specs());
+    specs.extend(quadratic_diophantine_equations::canonical_model_example_specs());
     specs.extend(sparse_matrix_compression::canonical_model_example_specs());
     specs
 }