let x be real number ; :: thesis: for r being real positive number holds (Ball (|[x,r]|,r)) \/ {|[x,0]|} is open Subset of Niemytzki-plane
let r be real positive number ; :: thesis: (Ball (|[x,r]|,r)) \/ {|[x,0]|} is open Subset of Niemytzki-plane
A1: r is Real by XREAL_0:def 1;
the carrier of Niemytzki-plane = y>=0-plane by Def3;
then reconsider a = |[x,0]| as Point of Niemytzki-plane by Th22;
consider BB being Neighborhood_System of Niemytzki-plane such that
A2: for x being Element of REAL holds BB . |[x,0]| = { ((Ball (|[x,q]|,q)) \/ {|[x,0]|}) where q is Element of REAL : q > 0 } and
for x, y being Element of REAL st y > 0 holds
BB . |[x,y]| = { ((Ball (|[x,y]|,q)) /\ y>=0-plane) where q is Element of REAL : q > 0 } by Def3;
x is Real by XREAL_0:def 1;
then BB . |[x,0]| = { ((Ball (|[x,q]|,q)) \/ {|[x,0]|}) where q is Element of REAL : q > 0 } by A2;
then (Ball (|[x,r]|,r)) \/ {|[x,0]|} in BB . a by A1;
hence (Ball (|[x,r]|,r)) \/ {|[x,0]|} is open Subset of Niemytzki-plane by YELLOW_8:12; :: thesis: verum