reserve a,b,c,d,e,f for Real,
        k,m for Nat,
        D for non empty set,
        V for non trivial RealLinearSpace,
        u,v,w for Element of V,
        p,q,r for Element of ProjectiveSpace(V);

theorem Th13:
  for n being Nat holds dom 1_Rmatrix(n) = Seg n
  proof
    let n be Nat;
    len 1_Rmatrix(n) = n by MATRIX_0:24;
    hence thesis by FINSEQ_1:def 3;
  end;
