A tensor is a mathematical object that can be thought of as a multi-dimensional array of numbers, which can be used to describe linear relationships between sets of vectors, scalars, and other tensors. Tensors can be classified into different types based on their rank, which is the number of indices required to describe them. Scalars are tensors of rank 0, vectors are tensors of rank 1, and matrices are tensors of rank 2.

Interestingly, Einstein originally struggled with the math needed for his theory. He famously had to be mentored in Ricci Calculus (tensor calculus) to finally formulate the field equations that describe how gravity warps spacetime.

Hence, the plea for a PDF is not about vanity; it is about a student spending 3 hours trying to understand a theorem that is literally illegible in their downloaded copy.

His book, Tensor Calculus , has been a staple for B.Sc. (Honors) and M.Sc. students in Indian universities (e.g., Calcutta University, Delhi University, and the UGC-CSIR NET syllabus) for over three decades.