已知以a为首项的数列满足an+1=an-3(当an>3) 2an(当an<=3)
若a,k为正整数,求使an+k=an对任意正整数n度成立的k与a;
若a=3\[(2^m)-1](m为正整数),试求数列an的前4m+2项的和s4m+2