let f be PartFunc of REAL,REAL; :: thesis: ( f is constant implies f is continuous )
assume A1: f is constant ; :: thesis: f is continuous

now :: thesis: for x0, r being Real st x0 in dom f & 0 < r holds
ex s being Real st
( 0 < s & ( for x1 being Real st x1 in dom f & |.(x1 - x0).| < s holds
|.((f . x1) - (f . x0)).| < r ) )

reconsider s = 1 as Real ;
let x0, r be Real; :: thesis: ( x0 in dom f & 0 < r implies ex s being Real st
( 0 < s & ( for x1 being Real st x1 in dom f & |.(x1 - x0).| < s holds
|.((f . x1) - (f . x0)).| < r ) ) )
assume that
A2: x0 in dom f and
A3: 0 < r ; :: thesis: ex s being Real st
( 0 < s & ( for x1 being Real st x1 in dom f & |.(x1 - x0).| < s holds
|.((f . x1) - (f . x0)).| < r ) )
take s = s; :: thesis: ( 0 < s & ( for x1 being Real st x1 in dom f & |.(x1 - x0).| < s holds
|.((f . x1) - (f . x0)).| < r ) )
thus 0 < s ; :: thesis: for x1 being Real st x1 in dom f & |.(x1 - x0).| < s holds
|.((f . x1) - (f . x0)).| < r
let x1 be Real; :: thesis: ( x1 in dom f & |.(x1 - x0).| < s implies |.((f . x1) - (f . x0)).| < r )
assume A4: x1 in dom f ; :: thesis: ( |.(x1 - x0).| < s implies |.((f . x1) - (f . x0)).| < r )
assume |.(x1 - x0).| < s ; :: thesis: |.((f . x1) - (f . x0)).| < r
f . x1 = f . x0 by A1, A2, A4;
hence |.((f . x1) - (f . x0)).| < r by A3, ABSVALUE:2; :: thesis: verum

end;

then f | (dom f) is continuous by Th14;
hence f is continuous ; :: thesis: verum