hi @eleanorkim , yes i know very interesting and specific.
your right they tend to move 40-60 years in a sinusoidal (wave) like pattern. His theroy is correct, but flawed like anything in life 
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for instance, some believe we are in the sixth wave that started in 2005. I do believe this to some extent.
this sixth wave can consist of the introduction to AI (chatgpt) ASI (Artificial super intelligence), advancement of health-care (new cures) and potentially energy (nuclear ect…) , In my opinion that is the sixth wave.
Kondratieff’s theory is flawed in a way that the future is unpredictable for example no one knew about Russia V Ukraine war , or Israel V palestine (i know the war has been going on since 1970’s somewhat around there but im talking about this new spark), Covid-19. Theres many examples.
I find it interesting how he was once a communist and was researching in a capitalist point of view, he got executed because of this.
these waves reflect the business cycle, i always like researching economists and mathematicians that have some involement with the economy or trading