Merge pull request #627 from SciML/s/fix-derivatives

Fix a case in expand_derivatives

Author: shashi

src/differentials.jl

@@ -119,9 +119,10 @@ function expand_derivatives(O::Term, simplify=true; occurances=nothing)
         if isempty(exprs)
             return c
         elseif length(exprs) == 1
-            return simplify ? SymbolicUtils.simplify(exprs[1]) : exprs[1]
+            term = (simplify ? SymbolicUtils.simplify(exprs[1]) : exprs[1])
+            return _iszero(c) ? term : c + term
         else
-            x = make_operation(+, !iszero(c) ? vcat(c, exprs) : exprs)
+            x = make_operation(+, !_iszero(c) ? vcat(c, exprs) : exprs)
             return simplify ? SymbolicUtils.simplify(x) : x
         end
     elseif !hasderiv(O)

test/derivatives.jl

@@ -110,3 +110,6 @@ L = .5 * ∂ₜ(x)^2 - .5 * x^2
 @test isequal(expand_derivatives(∂ₓ(L)), -1 * x)
 test_equal(expand_derivatives(Differential(x)(L) - ∂ₜ(Differential(∂ₜ(x))(L))), -1 * (∂ₜ(∂ₜ(x)) + x))
 @test isequal(expand_derivatives(Differential(x)(L) ~ ∂ₜ(Differential(∂ₜ(x))(L))), (-1 * x) ~ ∂ₜ(∂ₜ(x)))
+
+@variables x2(t)
+@test isequal(expand_derivatives(Differential(x)(2 * x + x2 * x)), 2 + x2)