public static List<BigInteger> primeFactorBig(BigInteger a){
    List<BigInteger> ans = new LinkedList<BigInteger>();

    for(BigInteger divisor = BigInteger.valueOf(2);
        a.compareTo(ONE) > 0; divisor = divisor.add(ONE))
        while(a.mod(divisor).equals(ZERO)){
             ans.add(divisor);
             a = a.divide(divisor);
        }
    return ans;
}