for (int bits = 5; bits < 32; bits++) {
      for (int i = 0; i < 100; i++) {
        long p = BigInteger.probablePrime(bits, rand).longValue();
        long q = BigInteger.probablePrime(bits, rand).longValue();
        assertFalse(LongMath.isPrime(p * q));
      }
    }
  }

  @GwtIncompatible // isPrime is GWT-incompatible
  public void testIsPrimeThrowsOnNegative() {