reserve x, r for Real;
reserve A for symmetrical Subset of COMPLEX;
reserve F,G for PartFunc of REAL, REAL;

theorem Th17:
  F is_even_on A implies -F is_even_on A
proof
  assume
A1: F is_even_on A;
  then
A2: A c= dom F;
  then
A3: A c= dom (-F) by VALUED_1:8;
  then
A4: dom((-F)|A) = A by RELAT_1:62;
A5: F|A is even by A1;
  for x st x in dom((-F)|A) & -x in dom((-F)|A) holds (-F)|A.(-x)=(-F)|A.x
  proof
    let x;
    assume that
A6: x in dom((-F)|A) and
A7: -x in dom((-F)|A);
A8: x in dom(F|A) by A2,A4,A6,RELAT_1:62;
A9: -x in dom(F|A) by A2,A4,A7,RELAT_1:62;
      reconsider x as Element of REAL by XREAL_0:def 1;
    (-F)|A.(-x)=(-F)|A/.(-x) by A7,PARTFUN1:def 6
      .=(-F)/.(-x) by A3,A4,A7,PARTFUN2:17
      .=(-F).(-x) by A3,A7,PARTFUN1:def 6
      .=-F.(-x) by VALUED_1:8
      .=-F/.(-x) by A2,A7,PARTFUN1:def 6
      .=-F|A/.(-x) by A2,A4,A7,PARTFUN2:17
      .=-F|A.(-x) by A9,PARTFUN1:def 6
      .=-F|A.x by A5,A8,A9,Def3
      .=-F|A/.x by A8,PARTFUN1:def 6
      .=-F/.x by A2,A4,A6,PARTFUN2:17
      .=-F.x by A2,A6,PARTFUN1:def 6
      .=(-F).x by VALUED_1:8
      .=(-F)/.x by A3,A6,PARTFUN1:def 6
      .=(-F)|A/.x by A3,A4,A6,PARTFUN2:17
      .=(-F)|A.x by A6,PARTFUN1:def 6;
    hence thesis;
  end;
  then (-F)|A is with_symmetrical_domain quasi_even by A4;
  hence thesis by A3;
end;
