286322
Bloch 's theorem proves that the solutions to this differential equation can be written as follows : where k is a vector ( called the wavevector ) , n is a discrete index ( called the band index ) , and un , k is a function with the same periodicity as the crystal lattice .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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