theorem Th45:
  |.1*n.| = sqrt n
proof
  reconsider j=1^2 as Element of REAL by XREAL_0:def 1;
  reconsider f= n |-> j as FinSequence of REAL;
  thus |.1*n .| = sqrt Sum f by RVSUM_1:56
    .= sqrt (n*1) by RVSUM_1:80
    .= sqrt n;
end;
