\[\left(\left(-2 \cdot J\right) \cdot \cos \left(\frac{K}{2}\right)\right) \cdot \sqrt{1 + {\left(\frac{U}{\left(2 \cdot J\right) \cdot \cos \left(\frac{K}{2}\right)}\right)}^{2}}\]

\left(\left(-2 \cdot J\right) \cdot \cos \left(\frac{K}{2}\right)\right) \cdot \sqrt{1 + {\left(\frac{U}{\left(2 \cdot J\right) \cdot \cos \left(\frac{K}{2}\right)}\right)}^{2}}

(FPCore (J K U)
 :precision binary64
 (*
  (* (* -2.0 J) (cos (/ K 2.0)))
  (sqrt (+ 1.0 (pow (/ U (* (* 2.0 J) (cos (/ K 2.0)))) 2.0)))))

double code(double J, double K, double U) {
	return ((-2.0 * J) * cos(K / 2.0)) * sqrt(1.0 + pow((U / ((2.0 * J) * cos(K / 2.0))), 2.0));
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2021060 
(FPCore (J K U)
  :name "Maksimov and Kolovsky, Equation (3)"
  :precision binary64
  (* (* (* -2.0 J) (cos (/ K 2.0))) (sqrt (+ 1.0 (pow (/ U (* (* 2.0 J) (cos (/ K 2.0)))) 2.0)))))

Backtrace

Failed to produce proof for (pow.f64 (sqrt.f64 (/.f64 U (.f64 (.f64 2 J) (cos.f64 (/.f64 K 2))))) 2) to (/.f64 (/.f64 U (*.f64 2 J)) (cos.f64 (/.f64 K 2)))		L	C
for-loop	/data/pavpan/nightlies/herbie/egg-generate-proof/src/core/simplify.rkt	167	8
(unnamed)	/data/pavpan/nightlies/herbie/egg-generate-proof/src/core/simplify.rkt	149	6
egraph-add-exprs	/data/pavpan/nightlies/herbie/egg-generate-proof/egg-herbie/main.rkt	154	0
with-egraph	/data/pavpan/nightlies/herbie/egg-generate-proof/egg-herbie/main.rkt	74	0
(unnamed)	/data/pavpan/nightlies/herbie/egg-generate-proof/src/core/simplify.rkt	79	0
simplify!	/data/pavpan/nightlies/herbie/egg-generate-proof/src/mainloop.rkt	236	0
run-iter!	/data/pavpan/nightlies/herbie/egg-generate-proof/src/mainloop.rkt	360	0
run-improve	/data/pavpan/nightlies/herbie/egg-generate-proof/src/mainloop.rkt	377	0
(unnamed)	/data/pavpan/nightlies/herbie/egg-generate-proof/src/sandbox.rkt	54	6
run	/opt/racket-7.9/share/pkgs/profile-lib/main.rkt	39	2
profile-thunk	/opt/racket-7.9/share/pkgs/profile-lib/main.rkt	9	0
(unnamed)	/opt/racket-7.9/collects/racket/engine.rkt	42	24