How to calculate complex exponential in shell script

Question

Can some one tell me what is wrong with this piece of code

read n 

for ((i=1;i<=n;i++))
  do 
   a= $((16/2**$((i-1))))

    for ((j=1;j<=a;j++))
      do 
        echo "1"
      done 

done 

When I executed it , i got error line 5: 16: command not found.