public Stream<BigDecimal> rootSequence() {
    return Stream.iterate(2, n -> n + 4)        // 2, 6, 10, 14, ...
        .flatMap(n -> Stream.of(n, -(n + 2)))   // 2, -4, 6, -8, 10, -12, 14, -16, ...
        .map(BigDecimal::valueOf);
}