pcubature

Multiple integration over convex polytopes.

Warning: the package does not work in GHCi.

---

This package allows to evaluate a multiple integral over a convex polytope. Let's consider for example the following integral:

\[\int_0^1\int_0^1\int_0^1 \exp(x+y+z)\,\text{d}z\,\text{d}y\,\text{d}x = {(e-1)}^3 \approx 5.07321411177285.\]

The domain of integration is the cube \({[0,1]}^3\). In order to use the package, one has to provide the vertices of this cube:

integrateOnPolytope'
    :: (Vector Double -> Double) -- ^ integrand
    -> [[Double]]                -- ^ vertices of the polytope
    -> Int                       -- ^ maximum number of evaluations
    -> Double                    -- ^ desired absolute error
    -> Double                    -- ^ desired relative error
    -> Int                       -- ^ integration rule: 1, 2, 3 or 4
    -> IO Result                 -- ^ value, error estimate, evaluations, success

Let's go:

module Main 
  where
import Numeric.Integration.PolyhedralCubature
import Data.Vector.Unboxed as V

f :: Vector Double -> Double
f v = exp (V.sum v)

cube :: [[Double]]
cube = [
         [0, 0, 0]
       , [0, 0, 1]
       , [0, 1, 0]
       , [0, 1, 1]
       , [1, 0, 0]
       , [1, 0, 1]
       , [1, 1, 0]
       , [1, 1, 1]
       ]

integral :: IO Result
integral = integrateOnPolytope' f cube 10000 0 1e-6 3

main :: IO ()
main = do 
  i <- integral
  print i
-- Result {
--          value = 5.073214090351428
--        , errorEstimate = 2.8421152805879766e-6
--        , evaluations = 710
--        , success = True
--        }

This cube is axis-aligned. So it may be better to use the adaptive-cubature package here. The pcubature package allows to evaluate multiple integrals whose bounds are (roughly speaking) linear combinations of the variables, such as:

\[\int_{-5}^4\int_{-5}^{3-x}\int_{-10}^{6-2x-y} f(x, y, z)\,\text{d}z\,\text{d}y\,\text{d}x.\]

Here, the domain of integration is given by the set of linear inequalities:

\[\left\{\begin{matrix} -5 & \leq & x & \leq & 4 \\\ -5 & \leq & y & \leq & 3-x \\\ -10 & \leq & z & \leq & 6-2x-y \end{matrix}\right.\]

Each of these linear inequalities defines a halfspace of \(\mathbb{R}^3\), and the intersection of these six halfspaces is a convex polytope (a polyhedron).

But it is not easy to get the vertices of this polytope. This is why the pcubature package depends on the vertexenum package, whose purpose is to enumerate the vertices of a polytope given as above, with linear inequalities. Let's take as example the function \(f(x,y,z) = x(x+1) - yz^2\):

module Main
  where
import Numeric.Integration.PolyhedralCubature
import Geometry.VertexEnum
import Data.VectorSpace     ( 
                              AdditiveGroup((^+^), (^-^))
                            , VectorSpace((*^)) 
                            )
import Data.Vector.Unboxed  as V

f :: Vector Double -> Double
f v = x * (x+1) - y * z * z
  where
    x = v ! 0
    y = v ! 1
    z = v ! 2

polytope :: [Constraint Double]
polytope = [
             x .>= (-5)         -- shortcut for `x .>=. cst (-5)`
           , x .<=  4
           , y .>= (-5)
           , y .<=. cst 3 ^-^ x -- we need `cst` here
           , z .>= (-10)
           , z .<=. cst 6 ^-^ 2*^x ^-^ y 
           ]
           where
             x = newVar 1
             y = newVar 2
             z = newVar 3

integral :: IO Result
integral = integrateOnPolytope' f polytope 10000 0 1e-6 3

main :: IO ()
main = do 
  i <- integral
  print i
-- Result {
--          value = 74321.77499999988
--        , errorEstimate = 1.0533262499999988e-7
--        , evaluations = 330
--        , success = True
--        }

The exact value of this integral is \(74321.775\), as we shall see later.

The function \(f\) of this example is polynomial. So we can use the function integratePolynomialOnPolytope to integrate it. This requires to define the polynomial with the help of the hspray package; we also import some modules of the numeric-prelude package, which allows to define a hspray polynomial more conveniently:

module Main
  where
import Numeric.Integration.PolyhedralCubature
import Geometry.VertexEnum
import Data.VectorSpace     ( 
                              AdditiveGroup((^+^), (^-^))
                            , VectorSpace((*^)) 
                            )
import Math.Algebra.Hspray  ( Spray, lone, (^**^) )
import Prelude hiding       ( (*), (+), (-) )
import qualified Prelude as P
import Algebra.Additive              
import Algebra.Module                
import Algebra.Ring

p :: Spray Double
p = x * (x + one) - (y * z^**^2) 
  where
    x = lone 1 :: Spray Double
    y = lone 2 :: Spray Double
    z = lone 3 :: Spray Double

polytope :: [Constraint Double]
polytope = [
             x .>= (-5)         -- shortcut for `x .>=. cst (-5)`
           , x .<=  4
           , y .>= (-5)
           , y .<=. cst 3 ^-^ x -- we need `cst` here
           , z .>= (-10)
           , z .<=. cst 6 ^-^ 2*^x ^-^ y 
           ]
           where
             x = newVar 1
             y = newVar 2
             z = newVar 3

integral :: IO Double
integral = integratePolynomialOnPolytope' p polytope

main :: IO ()
main = do 
  i <- integral
  print i
-- 74321.77499999967

The function integratePolynomialOnPolytope implements an exact procedure. However we didn't get the exact result. That's because of (small) numerical errors. The first numerical errors occur in the vertex enumeration performed by the vertexenum package:

module Main
  where
import Geometry.VertexEnum
import Data.VectorSpace     ( 
                              AdditiveGroup((^+^), (^-^))
                            , VectorSpace((*^)) 
                            )

polytope :: [Constraint Double]
polytope = [
             x .>= (-5)         
           , x .<=  4
           , y .>= (-5)
           , y .<=. cst 3 ^-^ x 
           , z .>= (-10)
           , z .<=. cst 6 ^-^ 2*^x ^-^ y 
           ]
           where
             x = newVar 1
             y = newVar 2
             z = newVar 3

vertices :: IO [[Double]]
vertices = vertexenum polytope Nothing

main :: IO ()
main = do 
  vs <- vertices
  print vs
-- [
--   [-5.000000000000003, 8.000000000000004, 8.000000000000004]
-- , [-4.999999999999998, -4.999999999999996, 20.999999999999993]
-- , [3.999999999999999, -0.9999999999999997, -1.0]
-- , [3.999999999999999, -5.0, 3.0000000000000004]
-- , [-5.0, -5.0, -10.0]
-- , [-5.0, 8.000000000000002, -10.0]
-- , [4.0, -0.9999999999999999, -10.0]
-- , [4.0, -5.0, -10.0]
-- ]

Since all coefficients of the linear inequalities are rational (they even are integral), the vertices should be rational as well. Unfortunately, vertexenum only allows to get vertices with double coordinates. So if we want to use Rational, we have to manually enter the vertices:

module Main
  where
import Numeric.Integration.PolyhedralCubature
import Math.Algebra.Hspray  ( Spray, lone, (^**^) )
import Prelude hiding       ( (*), (+), (-) )
import qualified Prelude as P
import Algebra.Additive              
import Algebra.Module                
import Algebra.Ring

p :: Spray Rational
p = x * (x + one) - (y * z^**^2) 
  where
    x = lone 1 :: Spray Rational
    y = lone 2 :: Spray Rational
    z = lone 3 :: Spray Rational

polytope :: [[Rational]]
polytope = [
             [-5, 8, 8]
           , [-5, -5, 21]
           , [4, -1, -1]
           , [4, -5, 3]
           , [-5, -5, -10]
           , [-5, 8, -10]
           , [4, -1, -10]
           , [4, -5, -10]
           ]

integral :: IO Rational
integral = integratePolynomialOnPolytope p polytope

main :: IO ()
main = do 
  i <- integral
  print i
-- 2972871 % 40

We get it, the exact value \(74321.775\), as promised.