function mycholesky(A)
    n,m=size(A)
    if n != m
        error("input is not a square matrix")
    end
    if A' != A
        error("input is not a symmetric matrix")
    end
    L=zeros(n,n)
    for i=1:n
        s=0
        for k=1:i-1
            s+=L[i,k]^2
        end
        if A[i,i]-s <= 0
            error("input is not positive definite")
        end
        L[i,i]=sqrt(A[i,i]-s)
        for j=i+1:n
            s=0
            for k=1:i-1
                s+=L[j,k]*L[i,k]
            end
            L[j,i]=(A[j,i]-s)/L[i,i]
        end
    end
    return L
end