theorem :: NFCONT_1:20

for X being set
for S being RealNormSpace
for f being PartFunc of the carrier of S,REAL holds
( f is_continuous_on X iff ( X c= dom f & ( for x0 being Point of S
for r being Real st x0 in X & 0 < r holds
ex s being Real st
( 0 < s & ( for x1 being Point of S st x1 in X & ||.(x1 - x0).|| < s holds
|.((f /. x1) - (f /. x0)).| < r ) ) ) ) )