# SymbolicNumericIntegration.jl

SymbolicNumericIntegration.jl is a hybrid symbolic/numerical integration package that works on the Julia Symbolics expressions.

SymbolicNumericIntegration.jl uses a randomized algorithm based on a hybrid of the method of undetermined coefficients and sparse regression and is able to solve a large subset of basic standard integrals (polynomials, exponential/logarithmic, trigonometric and hyperbolic, inverse trigonometric and hyperbolic, rational and square root). The basis of how it works and the theory of integration using the Symbolic-Numeric methods refer to Basis of Symbolic-Numeric Integration.

Function integrate returns the integral of a univariate expression with constant real or complex coefficients. integrate returns a tuple with three values. The first one is the solved integral, the second one is the sum of the unsolved terms, and the third value is the residual error. If integrate is successful, the unsolved portion is reported as 0.

using Symbolics
using SymbolicNumericIntegration

@variables x

julia> integrate(3x^3 + 2x - 5)
(x^2 + (3//4)*(x^4) - (5x), 0, 0)

julia> integrate((5 + 2x)^-1)
((1//2)*log((5//2) + x), 0, 0.0)

julia> integrate(1 / (6 + x^2 - (5x)))
(log(x - 3) - log(x - 2), 0, 3.339372764128952e-16)

y = integrate(1 / (x^2 - 16))
((1//8)*log(x - 4) - ((1//8)*log(4 + x)), 0, 1.546926788028958e-16)

julia> integrate(x^2/(16 + x^2))
(x + 4atan((-1//4)*x), 0, 1.3318788420751984e-16)

julia> integrate(x^2/sqrt(4 + x^2))
((1//2)*x*((4 + x^2)^0.5) - ((2//1)*log(x + sqrt(4 + x^2))), 0, 8.702422633074313e-17)

julia> integrate(x^2*log(x))
((1//3)*log(x)*(x^3) - ((1//9)*(x^3)), 0, 0)

julia> integrate(x^2*exp(x))
(2exp(x) + exp(x)*(x^2) - (2x*exp(x)), 0, 0)

julia> integrate(tan(2x))
((-1//2)*log(cos(2x)), 0, 0)

julia> integrate(sec(x)*tan(x))
(cos(x)^-1, 0, 0)

julia> integrate(cosh(2x)*exp(x))
((2//3)*exp(x)*sinh(2x) - ((1//3)*exp(x)*cosh(2x)), 0, 7.073930088880992e-8)

julia> integrate(cosh(x)*sin(x))
((1//2)*sin(x)*sinh(x) - ((1//2)*cos(x)*cosh(x)), 0, 4.8956233716268386e-17)

julia> integrate(cosh(2x)*sin(3x))
(0.153845sinh(2x)*sin(3x) - (0.23077cosh(2x)*cos(3x)), 0, 4.9807620877373405e-6)

julia> integrate(log(log(x))*(x^-1))
(log(x)*log(log(x)) - log(x), 0, 0)

julia> integrate(exp(x^2))
(0, exp(x^2), Inf)    # as expected!

integrate has the form integrate(y; kw...) or integrate(y, x; kw...), where y is the integrand and the optional x is the variable of integration. The keyword parameters are:

• abstol (default 1e-6): the error tolerance to accept a solution.
• symbolic (default true): if true, pure symbolic integration is attempted first.
• bypass (default false): if true, the whole expression is considered at once and not per term.
• num_steps (default 2): one plus the number of expanded basis to check (if num_steps is 1, only the main basis is checked).
• num_trials (default 5): the number of attempts to solve the integration numerically for each basis set.
• show_basis (default false): print the basis set, useful for debugging. Only works if verbose is also set.
• homotopy (default: true as of version 0.7.0): uses the continuous Homotopy operators to generate the integration candidates.
• verbose (default false): if true, prints extra (and voluminous!) debugging information.
• radius (default 1.0): the starting radius to generate random test points.
• opt (default STLSQ(exp.(-10:1:0))): the optimizer passed to sparse_regression!.
• max_basis (default 110): the maximum number of expression in the basis.
• complex_plane (default true): random test points are generated on the complex plane (only over the real axis if complex_plane is false).

## Testing

test/runtests.jl contains a test suite of 160 easy to moderate test integrals (can be run by calling test_integrals). Currently, SymbolicNumericIntegration.jl solves more than 90% of its test suite.

Additionally, 12 test suites from the Rule-based Integrator (Rubi) are included in the /test directory. For example, we can test the first one as below (Axiom refers to the format of the test files)

  using SymbolicNumericIntegration
include("test/axiom.jl")  # note, you may need to use the correct path

L = convert_axiom(:Apostle)   # can also use L = convert_axiom(1)
test_axiom(L, false; bypass=false, verbose=false, homotopy=true)

The test suites description based on the header of the files in the Rubi directory are

nameidcomment
:Apostle1Tom M. Apostol - Calculus, Volume I, Second Edition (1967)
:Bondarenko2Vladimir Bondarenko Integration Problems
:Bronstein3Manuel Bronstein - Symbolic Integration Tutorial (1998)
:Charlwood4Kevin Charlwood - Integration on Computer Algebra Systems (2008)
:Hearn5Anthony Hearn - Reduce Integration Test Package
:Hebisch6Waldek Hebisch - email May 2013
:Jeffrey7David Jeffrey - Rectifying Transformations for Trig Integration (1997)
:Moses8Joel Moses - Symbolic Integration Ph.D. Thesis (1967)
:Stewart9James Stewart - Calculus (1987)
:Timofeev10A. F. Timofeev - Integration of Functions (1948)
:Welz11Martin Welz - posts on Sci.Math.Symbolic
:Webster12Michael Wester

## Citation

If you use SymbolicNumericIntegration.jl, please cite Symbolic-Numeric Integration of Univariate Expressions based on Sparse Regression:

@article{Iravanian2022,
author = {Shahriar Iravanian and Carl Julius Martensen and Alessandro Cheli and Shashi Gowda and Anand Jain and Julia Computing and Yingbo Ma and Chris Rackauckas},
doi = {10.48550/arxiv.2201.12468},
month = {1},
title = {Symbolic-Numeric Integration of Univariate Expressions based on Sparse Regression},
url = {https://arxiv.org/abs/2201.12468v2},
year = {2022},
}