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BLOG TO understand DFT well

This blog explains the dft very well , esp the mathematics concept of Co-relation used to find the dft.

http://practicalcryptography.com/miscellaneous/machine-learning/intuitive-guide-discrete-fourier-transform/

Also suggests this book for practical understanding of DFT.
https://www.amazon.com/gp/product/0137027419/ref=as_li_qf_sp_asin_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0137027419&linkCode=as2&tag=practiccrypto-20

Understanding Digital Signal Processing-- By Richard G.Lyons