let G be Go-board; :: thesis: Int (h_strip (G,(width G))) = { |[r,s]| where r, s is Real : (G * (1,(width G))) `2 < s }
0 <> len G by GOBOARD1:def 5;
then 1 <= len G by NAT_1:14;
then A1: h_strip (G,(width G)) = { |[r,s]| where r, s is Real : (G * (1,(width G))) `2 <= s } by GOBOARD5:7;
thus Int (h_strip (G,(width G))) c= { |[r,s]| where r, s is Real : (G * (1,(width G))) `2 < s } :: according to XBOOLE_0:def 10 :: thesis: { |[r,s]| where r, s is Real : (G * (1,(width G))) `2 < s } c= Int (h_strip (G,(width G)))

proof

let x be set ; :: according to TARSKI:def 3 :: thesis: ( not x in Int (h_strip (G,(width G))) or x in { |[r,s]| where r, s is Real : (G * (1,(width G))) `2 < s } )
assume A2: x in Int (h_strip (G,(width G))) ; :: thesis: x in { |[r,s]| where r, s is Real : (G * (1,(width G))) `2 < s }
then reconsider u = x as Point of (Euclid 2) by Lm6;
consider s1 being real number such that
A3: s1 > 0 and
A4: Ball (u,s1) c= h_strip (G,(width G)) by A2, Th8;
reconsider p = u as Point of (TOP-REAL 2) by Lm6;
A5: p = |[(p `1),(p `2)]| by EUCLID:57;
reconsider s1 = s1 as Real by XREAL_0:def 1;
set q = |[((p `1) + 0),((p `2) - (s1 / 2))]|;
s1 / 2 < s1 by A3, XREAL_1:218;
then |[((p `1) + 0),((p `2) - (s1 / 2))]| in Ball (u,s1) by A3, A5, Th13;
then |[((p `1) + 0),((p `2) - (s1 / 2))]| in h_strip (G,(width G)) by A4;
then ex r2, s2 being Real st
( |[((p `1) + 0),((p `2) - (s1 / 2))]| = |[r2,s2]| & (G * (1,(width G))) `2 <= s2 ) by A1;
then (G * (1,(width G))) `2 <= (p `2) - (s1 / 2) by SPPOL_2:1;
then A6: ((G * (1,(width G))) `2) + (s1 / 2) <= p `2 by XREAL_1:21;
(G * (1,(width G))) `2 < ((G * (1,(width G))) `2) + (s1 / 2) by A3, XREAL_1:31, XREAL_1:217;
then (G * (1,(width G))) `2 < p `2 by A6, XXREAL_0:2;
hence x in { |[r,s]| where r, s is Real : (G * (1,(width G))) `2 < s } by A5; :: thesis: verum

end;

let x be set ; :: according to TARSKI:def 3 :: thesis: ( not x in { |[r,s]| where r, s is Real : (G * (1,(width G))) `2 < s } or x in Int (h_strip (G,(width G))) )
assume x in { |[r,s]| where r, s is Real : (G * (1,(width G))) `2 < s } ; :: thesis: x in Int (h_strip (G,(width G)))
then consider r, s being Real such that
A7: x = |[r,s]| and
A8: (G * (1,(width G))) `2 < s ;
reconsider u = |[r,s]| as Point of (Euclid 2) by TOPREAL3:13;
A9: Ball (u,(s - ((G * (1,(width G))) `2))) c= h_strip (G,(width G))

proof

let y be set ; :: according to TARSKI:def 3 :: thesis: ( not y in Ball (u,(s - ((G * (1,(width G))) `2))) or y in h_strip (G,(width G)) )
A10: Ball (u,(s - ((G * (1,(width G))) `2))) = { v where v is Point of (Euclid 2) : dist (u,v) < s - ((G * (1,(width G))) `2) } by METRIC_1:18;
assume y in Ball (u,(s - ((G * (1,(width G))) `2))) ; :: thesis: y in h_strip (G,(width G))
then consider v being Point of (Euclid 2) such that
A11: v = y and
A12: dist (u,v) < s - ((G * (1,(width G))) `2) by A10;
reconsider q = v as Point of (TOP-REAL 2) by TOPREAL3:13;
0 <= (r - (q `1)) ^2 by XREAL_1:65;
then ( (s - (q `2)) ^2 >= 0 & ((s - (q `2)) ^2) + 0 <= ((r - (q `1)) ^2) + ((s - (q `2)) ^2) ) by XREAL_1:8, XREAL_1:65;
then A13: sqrt ((s - (q `2)) ^2) <= sqrt (((r - (q `1)) ^2) + ((s - (q `2)) ^2)) by SQUARE_1:94;
A14: q = |[(q `1),(q `2)]| by EUCLID:57;
then sqrt (((r - (q `1)) ^2) + ((s - (q `2)) ^2)) < s - ((G * (1,(width G))) `2) by A12, Th9;
then sqrt ((s - (q `2)) ^2) <= s - ((G * (1,(width G))) `2) by A13, XXREAL_0:2;
then A15: abs (s - (q `2)) <= s - ((G * (1,(width G))) `2) by COMPLEX1:158;

per cases ( s >= q `2 or s <= q `2 ) ;

suppose s >= q `2 ; :: thesis: y in h_strip (G,(width G))

then s - (q `2) >= 0 by XREAL_1:50;
then abs (s - (q `2)) = s - (q `2) by ABSVALUE:def 1;
then (G * (1,(width G))) `2 <= q `2 by A15, XREAL_1:12;
hence y in h_strip (G,(width G)) by A1, A11, A14; :: thesis: verum

end;

suppose s <= q `2 ; :: thesis: y in h_strip (G,(width G))

then (G * (1,(width G))) `2 <= q `2 by A8, XXREAL_0:2;
hence y in h_strip (G,(width G)) by A1, A11, A14; :: thesis: verum

end;

end;

end;

reconsider B = Ball (u,(s - ((G * (1,(width G))) `2))) as Subset of (TOP-REAL 2) by TOPREAL3:13;
A16: B is open by Th6;
u in Ball (u,(s - ((G * (1,(width G))) `2))) by A8, Th4, XREAL_1:52;
hence x in Int (h_strip (G,(width G))) by A7, A9, A16, TOPS_1:54; :: thesis: verum