theorem Th13:
  Center G <= len G
proof
  0 < len G by GOBRD11:34;
  then
A1: len G div (2 qua Integer) < len G by INT_1:56;
  Center G = len G div 2 + 1 by JORDAN1A:def 1;
  hence thesis by A1,NAT_1:13;
end;
