sos

Harrison's sum-of-squares decision procedure for nonlinear real arithmetic, in Lean 4. Based on the design from Harrison 2007 (TPHOLs).

Adding to your project

Add the following to your lakefile.toml:

[[require]]
name = "sos"
git = "https://github.com/leanprover/sos.git"
rev = "main"

Or, in a lakefile.lean:

require sos from git
  "https://github.com/leanprover/sos.git" @ "main"

Then import SOS and use by sos. The system BLAS/LAPACK runtime must be installed; see Native dependency troubleshooting.

Status

The by sos tactic closes nonlinear-real-arithmetic goals end-to-end: reify the goal, encode an SDP, call CSDP, round the float Gram matrix to rationals, decompose via LDLᵀ + Lagrange four-square, and dispatch the matching verifier-soundness lemma.

Examples

A curated version of this section lives in SOSTest/Showcase.lean, so lake test checks that these public examples continue to elaborate.

import SOS

-- Cauchy–Schwarz (rank 1, deg 4, 4 vars)
example (a b c d : ℝ) :
    0 ≤ (a^2 + b^2) * (c^2 + d^2) - (a*c + b*d)^2 := by sos

-- Cyclic Schur, 3 vars
example (a b c : ℝ) : 0 ≤ a^2 + b^2 + c^2 - a*b - b*c - a*c := by sos

-- AM ≥ GM squared
example (x y : ℝ) : 0 ≤ (x^2 + y^2)^2 - 4*x^2*y^2 := by sos

-- Strict positivity, multivariate
example (x y : ℝ) : 0 < x^2 + y^2 + 1 := by sos

-- Infeasibility
example (x : ℝ) : ¬ (x^4 + 1 ≤ 0) := by sos

-- Constrained
example (x : ℝ) (_h : 0 ≤ x) : 0 ≤ x^3 + x := by sos
example (x y : ℝ) (_hx : 0 ≤ x) (_hy : 0 ≤ y) :
    0 ≤ x^2 + 2*x*y + y^2 := by sos

-- Strict-inequality hypothesis (promoted to `0 ≤ x` via `le_of_lt`)
example (x : ℝ) (_h : 0 < x) : 0 ≤ x^3 + x := by sos

-- Equality constraint: discriminant of a real-rooted quadratic
example (a b c x : ℝ) (_h : a*x^2 + b*x + c = 0) :
    0 ≤ b^2 - 4*a*c := by sos

-- Discrete goal, lifted/refuted through ℝ
example : ∀ n : ℕ, n ≤ n * n := by sos

-- Euclidean division over ℕ
example : ∀ a b : ℕ, b ≠ 0 → a = b * (a / b) + a % b := by sos

by sos? reports the witness it found as a Try this: suggestion that you can paste back as a sos_witness … invocation, freezing the proof so it no longer depends on calling CSDP at compile time:

example (x : ℝ) : 0 ≤ x^2 + 1 := by sos?
-- Try this:
--   [apply] sos_witness
--     { sigmas := [([], { terms := [((1 : ℚ), CMvPolynomial.C (1 : ℚ)),
--                                   ((1 : ℚ), CMvPolynomial.X 0)] })] }

Witness terms are (c, p) pairs interpreted as c · p², with 0 ≤ c enforced by Certificate.checks.

Atoms are recovered as arbitrary ℝ-typed subterms (free variables, function applications, projections — anything the reifier doesn't recognise as a known operator). Constraint hypotheses can come from the local context or from →-introduced binders.

The Motzkin polynomial x⁴y² + x²y⁴ + 1 - 3x²y² is non-negative but not a sum of squares (Hilbert 1888 / Motzkin 1967), so plain by sos has no certificate to find and correctly fails, caught here by fail_if_success:

example : True := by
  fail_if_success
    (have : ∀ x y : ℝ,
        0 ≤ x^4 * y^2 + x^2 * y^4 + 1 - 3*x^2*y^2 := by sos)
  trivial

Opting into the Positivstellensatz power refutation with maxRefutationPower closes it anyway — by sos negates the goal to 0 < -M, searches for -M = σ₀ + σ₁·(-M) (Harrison's REAL_NONLINEAR_PROVER), and recovers the exact rational certificate from CSDP's rank-deficient solution by facial reduction:

example (x y : ℝ) : 0 ≤ x^4*y^2 + x^2*y^4 + 1 - 3*x^2*y^2 := by
  sos (config := { maxRefutationPower := 1 })

This is off by default because each power is a family of growing-degree CSDP solves whose required degree is not known in advance; see Scope and limits.

The soundness lemmas reduce to IsSumSq.nonneg (Mathlib) once the goal has been transported through the aeval ring-hom on HexMvPoly's CMvPolynomial n ℚ compatibility façade. Two design points worth flagging, both following Harrison's [TPHOLs 2007 paper] (https://link.springer.com/chapter/10.1007/978-3-540-74591-4_9):

• min tr(X) cost matrix. CSDP's interior-point step has no preferred direction on a singleton boundary feasible set, so we give it the trace objective Harrison reports works empirically.
• Zero-pivot LDLᵀ. Rank-deficient SOS Grams ((x + y)² has Gram [[1,1],[1,1]], rank 1) require the "completing the square" routine to accept a zero pivot when the residual column is also zero. Our SOS.LDL.decompose does this; LDL.reconstruct already drops the zero-D contributions.

Implementation notes

The computational engine depends directly on the Mathlib-free Hex.MvPoly library. Certificate checking reduces against its kernel-decidable representation. The small Mathlib-facing layer under SOS/Mathlib/ supplies only the real interpretation and operation laws needed by the soundness proofs; SOS does not depend on the general hex-mv-poly-mathlib correspondence package.

Scope and limits

• Rational certificates only. Witnesses live in CMvPolynomial n ℚ throughout. CSDP returns floats, which we round against a denominator schedule (Harrison's find_rounding exactly: the integers [1..31], then the powers of two 2^5 … 2^66, capped per-call by Config.maxRoundingDenomLog2, the base-2 log of the cap, default 66) and decompose via rational LDLᵀ. Each σ-block is a weighted sum of squares Σᵢ cᵢ · pᵢ² with cᵢ ∈ ℚ≥0; the non-negativity is decide +kernel-checked at certificate time. There is no support for algebraic-extension coefficients — a goal whose only SOS witness involves √2 (or any other irrational) is out of reach by construction.

• Putinar form, with hypotheses as multipliers. A certificate is target = σ₀ + Σᵢ σᵢ · gᵢ with each σᵢ an SOS, where the gᵢ are non-negativity hypotheses pulled from intro-binders and the local context. Recognised constraint shapes are 0 ≤ g, g ≤ 0 (encoded as 0 ≤ −g), and 0 < g (used via le_of_lt). Unconstrained goals reduce to target = σ₀. Strict positivity 0 < p is handled by an LP-slack maximisation: one extra decision variable λ ≥ 0 enters the SDP via the constant-monomial equality, CSDP maximises it to discover the largest admissible slack λ*, and then ε = 2^-k near λ* is fed to the standard feasibility pipeline to produce the verifiable certificate. Infeasibility uses target = −1.

• Single fixed relaxation level by default. Multiplier basis sizes are set once from a degree bound D = max(deg(target), maxᵢ deg(gᵢ)): the σ₀ basis is monomials up to ⌈D/2⌉, and each σᵢ basis is monomials up to ⌈max(0, D − deg(gᵢ))/2⌉. The default search runs no hierarchy walk that bumps the relaxation order on failure, so a goal needing a higher order is left open.

• Non-SOS non-negative polynomials, opt-in. A polynomial that is non-negative but not a sum of squares (Motzkin x⁴y² + x²y⁴ + 1 − 3x²y² is the canonical example) has no direct certificate, so the default search fails. Setting maxRefutationPower := k enables Harrison's REAL_NONLINEAR_PROVER power refutation: for i = 1 … k it searches for a certificate of −(−p)^i against {−p ≥ 0} (negate the goal, then refute 0 < −p), and recovers the exact rational certificate from CSDP's rank-deficient boundary solution by facial reduction — rationalising the null-space projector of the float Gram and imposing it as exact linear constraints. This is off by default: each power is a family of growing-degree CSDP solves, the required degree is unknown a priori, and the recovery adds a symmetric eigendecomposition per block.

• Search failure is not a soundness failure. When CSDP returns an unusable status, when no rounding denominator validates, or when LDLᵀ reconstruction can't close the certificate, by sos reports "no certificate found" and leaves the goal open. The Certificate.checks predicate that closes the goal is decide +kernel-checked against Certificate n data, so a proof that goes through is independent of CSDP correctness.

Building and testing

This repository is pinned to the Lean version in lean-toolchain; dependencies are pinned by lake-manifest.json.

git clone https://github.com/leanprover/sos
cd sos
lake exe cache get
lake test

lake test elaborates SOSTest, which runs by sos against every example in SOSTest/Examples.lean — each invocation calls CSDP, rounds the Gram matrix, reconstructs the certificate, and checks it. A passing lake test is end-to-end verification of the search/round/reconstruct/verify pipeline.

Native dependency troubleshooting

The tactic calls CSDP through csdp-ffi, so BLAS/LAPACK must be available before Lean can load the native solver. The csdp-ffi package owns the resolved native artifacts and propagates them to SOS and its downstream consumers; downstream lakefiles do not need CSDP, BLAS, or LAPACK link flags. If lake build or lake test fails while compiling or linking native code, run:

(cd .lake/packages/CSDP && lake script run checkNativeDeps)

That preflight reports the platform-specific packages or SDK paths expected by the native build. For examples that invoke CSDP, prefer Lake targets such as lake test. A direct Lean invocation must receive the setup file generated by Lake (for example, lake env lean SomeFile.lean --setup=path/to/setup.json).

Architecture

The tactic runs three stages on a by sos goal:

1. SOS.Reify.parseGoalAtomic in SOS.Mathlib.Reify walks the goal expression, collecting atomic ℝ-typed subterms into an array and producing untyped SOS.Poly.Raw ASTs for the conclusion and each constraint hypothesis (drawn from intro-binders and the local context).
2. SOS.Engine.solve owns the complete search policy. It builds the Putinar-form SDP, calls CSDP, rounds the float Gram matrix to rationals, runs LDLᵀ, and returns a structured result covering every proof route.
3. SOS.Mathlib.Tactic reruns SOS.Engine.Result.check and discharges the real-arithmetic goal via the matching soundness lemma in SOS.Mathlib.Verifier.

Module	What it provides
SOS.Raw	Mathlib-free Poly.Raw and typed Poly n ASTs.
SOS.Polynomial	The narrow operational Hex.MvPoly API used by the engine.
SOS.Certificate	Goal n, SOSDecomp, Certificate n, checks predicate.
SOS.Core	Stable Mathlib-free data and executable-checking interface for proof-facing consumers.
SOS.LDL	Rational LDLᵀ and Gram→weighted-square reconstruction.
SOS.Search	Putinar-form SDP encoding, CSDP integration, rounding and reconstruction.
SOS.Engine	Native solver façade: public Config, Problem, Result, solve, and deterministic Result.check.
SOS.Mathlib.Raw	Interpretation of the raw and typed polynomial syntax over ℝ.
SOS.Mathlib.Certificate	Interpretation of certificate decompositions and checked identities over ℝ.
SOS.Mathlib.Polynomial	Narrow real interpretation and operation-preservation lemmas.
SOS.Mathlib.Verifier	Certificate/result soundness over ℝ.
SOS.Mathlib.Reify	Atom-collecting Lean-Expr walker → ParsedGoal.
SOS.Mathlib.Lift	Lifting and refutation bridges for ℕ, ℤ, and ℚ.
SOS.Mathlib.Tactic	by sos, by sos?, by pure_sos, and witness elaborators.
SOSTest.Examples	Worked examples invoking the tactic; serves as the lake test driver.
SOSTest.Showcase	Curated launch/demo examples that are also covered by lake test.

Dependencies

Package	Purpose
leanprover-community/mathlib4	Real semantics, soundness proofs, reification, lifting and tactic elaboration under SOS/Mathlib/.
leanprover/hex-mv-poly	Mathlib-free kernel-decidable multivariate-polynomial substrate.
leanprover/csdp-ffi	FFI wrapper around CSDP 6.2.0. Vendored CSDP source.

System dependencies (BLAS/LAPACK, transitively via csdp-ffi):

Platform	Packages
Linux	liblapack-dev libblas-dev gfortran
macOS	Apple Command Line Tools (Accelerate framework)
Windows	MSYS2 mingw-w64 with mingw-w64-x86_64-openblas

CI runs Linux-only.

Contributing

See CONTRIBUTING.md for development workflow and test expectations.

Licence

Apache License 2.0 (see LICENSE). Transitively, CSDP is distributed under the Eclipse Public License 1.0.

References

• John Harrison, "Verifying Nonlinear Real Formulas Via Sums of Squares" (TPHOLs 2007).
• Pablo Parrilo, Structured Semidefinite Programs and Semialgebraic Geometry (Caltech PhD, 2000).
• Helena Peyrl & Pablo Parrilo, "Computing sum of squares decompositions with rational coefficients" (Theor. Comput. Sci. 2008).
• Brian Borchers, CSDP.
• Coq Micromega psatz — design template.