View Compilation Error

0_0_26026154_11523.cpp: In function 'int main()':
0_0_26026154_11523.cpp:14:30: error: 'a' was not declared in this scope
         w[i] = (i-3) * (a(i-1)%mod)%mod + (i-2) * ((2*a(i-2)%mod + a(i-3)%mod)%mod)%mod;
                              ^