public static boolean isAntiarithmetic() {
		BitSet bs = new BitSet(a.length + 1);
		for (int i = 0; i < a.length; i++) {
			bs.clear();
			for (int j = i + 1; j < a.length; j++) {
				bs.set(a[j], true);
				if ((a[j] + a[i]) % 2 == 0 && bs.get((a[j] + a[i]) / 2))
					return false;
			}
		}
		return true;
	}