function IsPrime(N: Integer): Boolean;
var
  Max: Integer;     // max divisor
  Divisor: Integer; // different divisors to try
begin
  Result := False;
  if N < 2 then
    Exit; // not a prime
  Result := True;
  if N = 2 then
    Exit; // 2 is prime
  if N mod 2 = 0 then
    Result := False; // even numbers > 2 are not prime
  Max := Trunc(Sqrt(N)) + 1;
  Divisor := 3;
  // test odd numbers
  while (Max > Divisor) and Result do
  begin
    if (N mod Divisor) = 0 then
      Result := False;
    Inc(Divisor, 2); // next odd number
  end;
end;
//delphi/2278