reserve a,b,c,d,m,x,n,j,k,l for Nat,
  t,u,v,z for Integer,
  f,F for FinSequence of NAT;
reserve p,q,r,s for real number;
reserve a,b,c,d,m,x,n,k,l for Nat,
  t,z for Integer,
  f,F,G for FinSequence of REAL;
reserve q,r,s for real number;
reserve D for set;

theorem Th90:
  3 divides b or 3 divides b-1 or 3 divides b+1
  proof
    2*1+1 = 3 & b|^1 = b;
    hence thesis by Th70,PEPIN:41;
  end;
