a 1=0
When n≥2,
Sn = - an
S(n- 1)= - a(n- 1)
an=a(n- 1)-an
2an=a(n- 1)
0+a2= - a2
a2=0
an=( 1/2)a(n- 1)
an=0
The sequence {an} is a constant sequence.