Jacobian#jacobian

# File tmp/rubies/ruby-3.3.0/ext/bigdecimal/lib/bigdecimal/jacobian.rb, line 79
def jacobian(f,fx,x)
  n = x.size
  dfdx = Array.new(n*n)
  for i in 0...n do
    df = dfdxi(f,fx,x,i)
    for j in 0...n do
      dfdx[j*n+i] = df[j]
    end
  end
  dfdx
end