1,1,1,2,2,3,4,5,7,9,12,16,21找规律,第n个是多少

问题描述:

1,1,1,2,2,3,4,5,7,9,12,16,21找规律,第n个是多少
求本题公式 (能算出第100个的)

a(n+3)=an+a(n+1)