find missing numbers inside lots of files

I have a big list of ordered files with names like this

file_1.txt file_2.txt file_3.txt file_6.txt file_7.txt file_8.txt file_10.txt

In this case it is easy to see that files: file_4.txt,file_5.txt and file_9.txt are missing, but if i have a big list how can i find the missing files? i am just learning bash so i just know some simple examples. like this

for i in $(seq 1 1000) ;

do  
  if [i not in *.txt];  then                                                                                                     
  echo $i;

done

but this doesnt even work unless i erase the "if [i not in *.txt];then" line so it just writes all the numbers between 1 and 1000 i hope you can help me thanks in advanced Ce