Where to get “idf” coef for words?

Idf is Inverse Document Frequency. In other words, the frequency of the term goes in the denominator. So what you want are word frequency tables.

Wiktionary:Frequency lists should get you started. Keep in mind these lists treat inflected forms of a word as the same word e.g. Be, is, am, are, ....

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.