# Announcing support for complex-domain linear programs in Convex.jl

I am pleased to announce the support for complex-domain linear programs (LPs) in Convex.jl. As one of the Google Summer of Code students under The Julia Language, I had proposed to implement the support for complex semidefinite programming. In the first phase of project, I started by tackling the problem of complex-domain LPs where in first subphase, I had announced the support for complex coefficients during JuliaCon’16 and now I take this opportunity to announce the support for complex variables in LPs.

Complex-domain LPs consist of a real linear objective function, real linear inequality constraints, and real and complex linear equality constraints.

In order to enable complex-domain LPs, we came up with these ideas:

1. We redefined the conic_form! of every affine atom to accept complex arguments.
2. Every complex variable z was internally represented as z = z1 + i*z2, where z1 and z2 are real.
3. We introduced two new affine atoms real and imag which return the real and the imaginary parts of the complex variable respectively.
4. transpose and ctranspose perform differently on complex variables so a new atom CTransposeAtom was created.
5. A complex-equality constraint RHS = LHS can be decomposed into two corresponding real equalities constraint real(RHS) = real(LHS) and imag(RHS) = imag(LHS)

After above changes were made to the codebase, we wrote two use cases to demonstrate the usability and the correctness of our idea which I am presenting below:

# Importing Packages
Pkg.clone("https://github.com/Ayush-iitkgp/Convex.jl/tree/gsoc2")
using Convex

# Complex LP with real variable
n = 10 # variable dimension (parameter)
m = 5 # number of constraints (parameter)
xo = rand(n)
A = randn(m,n) + im*randn(m,n)
b = A * xo
# Declare a real variable
x = Variable(n)
p1 = minimize(sum(x), A*x == b, x>=0)
# Notice A*x==b is complex equality constraint
solve!(p1)
x1 = x.value

# Let's now solve by decomposing complex equality constraint into the corresponding real and imaginary part.
p2 = minimize(sum(x), real(A)*x == real(b), imag(A)*x==imag(b), x>=0)
solve!(p2)
x2 = x.value
x1==x2 # should return true

# Let's now consider an example using a complex variable
# Complex LP with complex variable
n = 10 # variable dimension (parameter)
m = 5 # number of constraints (parameter)
xo = rand(n)+im*rand(n)
A = randn(m,n) + im*randn(m,n)
b = A * xo

# Declare a complex variable
x = ComplexVariable(n)
p1 = minimize(real(sum(x)), A*x == b, real(x)>=0, imag(x)>=0)
solve!(p1)
x1 = x.value

xr = Variable(n)
xi = Variable(n)
p2 = minimize(sum(xr), real(A)*xr-imag(A)*xi == real(b), imag(A)*xr+real(A)*xi == imag(b), xr>=0, xi>=0)
solve!(p2)
x1== xr.value + im*xi.value # should return true


List of all the affine atoms are as follows:

1. addition, substraction, multiplication, division
2. indexing and slicing
3. k-th diagonal of a matrix
4. construct diagonal matrix
5. transpose and ctranspose
6. stacking
7. sum
8. trace
9. conv
10. real and imag

Now, I am working towards implementing complex-domain second order conic programming. Meanwhile, I invite the Julia community to play around with the complex-domain LPs. The link to the development branch is here.

Looking forward to your suggestions!

Special thanks to my mentors Madeleine Udell and Dvijotham Krishnamurthy!