Sledgehammering... 
"vampire": Error: The Vampire prover is not activated; to activate it, set the Isabelle system option
                  "vampire_noncommercial" to "yes" (e.g. via the Isabelle/jEdit menu Plugin Options / Isabelle /
                  General)
 
Proof found... 
"cvc4": Try this: by (simp add: power2_sum) (0.0 ms) 
"z3": Try this: by (simp add: power2_sum) (0.0 ms) 
"e": Timed out