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Bharata Natyam is from South India
Those who are writing Bharatnatyam is from Tamilnadu, eiether they do not know or they are referring to some articles written with lack of knowledge.

Referenece Books:

1. Bharatanatyam (Paperback)by Ashish Mohan Khokar (Author) ""Bharatanatyam is among the oldest and most popular of India's classical dance styles..."" (more) 

2. At Home in the World: Bharata Natyam on the Global Stage (Paperback)by Janet O'Shea  
Janet O'Shea (Author)"