let p, q be R_eal; :: thesis: for f being PartFunc of REAL,REAL st f is_differentiable_on ].p,q.[ & ( for x being Real st x in ].p,q.[ holds
diff (f,x) < 0 ) holds
f | ].p,q.[ is decreasing
let f be PartFunc of REAL,REAL; :: thesis: ( f is_differentiable_on ].p,q.[ & ( for x being Real st x in ].p,q.[ holds
diff (f,x) < 0 ) implies f | ].p,q.[ is decreasing )
assume that
A1: f is_differentiable_on ].p,q.[ and
A2: for x being Real st x in ].p,q.[ holds
diff (f,x) < 0 ; :: thesis: f | ].p,q.[ is decreasing

now :: thesis: for x1, x2 being Real st x1 in ].p,q.[ /\ (dom f) & x2 in ].p,q.[ /\ (dom f) & x1 < x2 holds
f . x2 < f . x1

let x1, x2 be Real; :: thesis: ( x1 in ].p,q.[ /\ (dom f) & x2 in ].p,q.[ /\ (dom f) & x1 < x2 implies f . x2 < f . x1 )
assume that
A3: ( x1 in ].p,q.[ /\ (dom f) & x2 in ].p,q.[ /\ (dom f) ) and
A4: x1 < x2 ; :: thesis: f . x2 < f . x1
A5: 0 < x2 - x1 by A4, XREAL_1:50;
reconsider Z = ].x1,x2.[ as open Subset of REAL ;
( x1 in ].p,q.[ & x2 in ].p,q.[ ) by A3, XBOOLE_0:def 4;
then A6: [.x1,x2.] c= ].p,q.[ by XXREAL_2:def 12;
then A7: f | [.x1,x2.] is continuous by A1, FDIFF_1:25, FCONT_1:16;
A8: Z c= [.x1,x2.] by XXREAL_1:25;
then f is_differentiable_on Z by A1, A6, FDIFF_1:26, XBOOLE_1:1;
then ex x0 being Real st
( x0 in ].x1,x2.[ & diff (f,x0) = ((f . x2) - (f . x1)) / (x2 - x1) ) by A1, A4, A6, A7, ROLLE:3, XBOOLE_1:1;
then (f . x2) - (f . x1) < 0 by A2, A5, A6, A8;
then f . x2 < (f . x1) + 0 by XREAL_1:19;
hence f . x2 < f . x1 ; :: thesis: verum

end;

hence f | ].p,q.[ is decreasing by RFUNCT_2:21; :: thesis: verum