What is the answer: The fifteen elements with atomic numbers 57 to 71, which includes Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Europium (Eu), Erbium (Er), Thulium (Tm), Ytterbium (Yb) and Lutetium (Lu), are known as what?
lanthanoid series