
theorem Th42:
  for S1, S2 being antisymmetric non empty RelStr for D being
  Subset of [:S1,S2:] holds ex_inf_of proj1 D,S1 & ex_inf_of proj2 D,S2 iff
  ex_inf_of D,[:S1,S2:]
proof
  let S1, S2 be antisymmetric non empty RelStr, D be Subset of [:S1,S2:];
A1: the carrier of [:S1,S2:] = [:the carrier of S1, the carrier of S2:] by Def2
;
  then
A2: D c= [:proj1 D,proj2 D:] by Th1;
  hereby
    assume that
A3: ex_inf_of proj1 D,S1 and
A4: ex_inf_of proj2 D,S2;
    ex a being Element of [:S1,S2:] st D is_>=_than a & for b being
    Element of [:S1,S2:] st D is_>=_than b holds a >= b
    proof
      consider x2 being Element of S2 such that
A5:   proj2 D is_>=_than x2 and
A6:   for b being Element of S2 st proj2 D is_>=_than b holds x2 >= b
      by A4,YELLOW_0:16;
      consider x1 being Element of S1 such that
A7:   proj1 D is_>=_than x1 and
A8:   for b being Element of S1 st proj1 D is_>=_than b holds x1 >= b
      by A3,YELLOW_0:16;
      take a = [x1,x2];
      thus D is_>=_than a
      proof
        let q be Element of [:S1,S2:];
        assume q in D;
        then consider q1, q2 being object such that
A9:     q1 in proj1 D and
A10:    q2 in proj2 D and
A11:    q = [q1,q2] by A2,ZFMISC_1:def 2;
        reconsider q2 as Element of S2 by A10;
        reconsider q1 as Element of S1 by A9;
        q1 >= x1 & q2 >= x2 by A7,A5,A9,A10;
        hence thesis by A11,Th11;
      end;
      let b be Element of [:S1,S2:] such that
A12:  D is_>=_than b;
A13:  b = [b`1,b`2] by A1,MCART_1:21;
      then proj2 D is_>=_than b`2 by A12,Th34;
      then
A14:  x2 >= b`2 by A6;
      proj1 D is_>=_than b`1 by A12,A13,Th34;
      then x1 >= b`1 by A8;
      hence thesis by A13,A14,Th11;
    end;
    hence ex_inf_of D,[:S1,S2:] by YELLOW_0:16;
  end;
  assume ex_inf_of D,[:S1,S2:];
  then consider x being Element of [:S1,S2:] such that
A15: D is_>=_than x and
A16: for b being Element of [:S1,S2:] st D is_>=_than b holds x >= b by
YELLOW_0:16;
A17: x = [x`1,x`2] by A1,MCART_1:21;
  then
A18: proj1 D is_>=_than x`1 by A15,Th34;
A19: proj2 D is_>=_than x`2 by A15,A17,Th34;
  for b being Element of S1 st proj1 D is_>=_than b holds x`1 >= b
  proof
    let b be Element of S1;
    assume proj1 D is_>=_than b;
    then D is_>=_than [b,x`2] by A19,Th34;
    then x >= [b,x`2] by A16;
    hence thesis by A17,Th11;
  end;
  hence ex_inf_of proj1 D,S1 by A18,YELLOW_0:16;
  for b being Element of S2 st proj2 D is_>=_than b holds x`2 >= b
  proof
    let b be Element of S2;
    assume proj2 D is_>=_than b;
    then D is_>=_than [x`1,b] by A18,Th34;
    then x >= [x`1,b] by A16;
    hence thesis by A17,Th11;
  end;
  hence thesis by A19,YELLOW_0:16;
end;
