# Tensor Calculus Lecture 14d: Non-hypersurfaces - Relationship Among Curvature Tensors 1

TLDRThis video script delves into the intricate relationship among curvatures derived from shift tensors in a double embedding scenario. The presenter meticulously applies covariant derivatives to explore tensors and invariants, emphasizing the importance of understanding the geometric implications of these mathematical constructs. The script guides viewers through the process of differentiating equations involving shift tensors, revealing the curvature normal tensor and its significance in tensor calculus, ultimately setting the stage for discussions on principal curvatures and further geometric insights in upcoming videos.

###### Takeaways

- π The video discusses the relationship among the shift tensors and curvatures in the context of a double embedding in a mathematical manifold.
- π The covariant derivative is the key operator used for differentiation in this context, leading to tensors and invariants.
- π The script emphasizes the importance of understanding the analogy between different manifolds and their respective alphabets for indices.
- π€ The presenter encourages viewers to train themselves to recognize the equivalence and properties of the covariant derivatives across different scenarios.
- π The application of the covariant derivative to both sides of the equations is a central part of the video, with careful attention to the use of the chain rule.
- π The left-hand side of the equations involves the curvature normal tensor, which is not yet an invariant but will become one after index contraction.
- π The product rule is used in the differentiation process, and the presenter hints at the complexity of dealing with the product rule in this context.
- π The chain rule is applied to the shift tensor with respect to the derivative on the curve, which is defined in the entire ambient space of the curve embedded in the surface.
- π The final relationship derived in the video is a second-order version that holds the key to the concept of principal curvatures and other geometric concepts.
- π― The relationship is very general and works in full generality with respect to the dimensionalities of the embeddings, applicable even in n-dimensional spaces.
- π The next steps involve contracting Greek indices to work towards invariants, with a focus on hypersurfaces to simplify the discussion and quickly reach the concept of principal curvatures.

###### Q & A

### What is the main focus of the video script?

-The main focus of the video script is to explore the relationship among the curvatures, which is derived from the relationship between the shift tensors in the context of a double embedding, using covariant derivatives.

### Why is the covariant derivative the appropriate operator to apply in this context?

-The covariant derivative is the appropriate operator to apply because it leads to tensors and eventually invariants, which are essential for understanding the geometric properties of the embedded manifolds.

### What does the script suggest about the shift tensor with a capital Greek index?

-The script suggests that the shift tensor with a capital Greek index may look unfamiliar at first, but it emphasizes the importance of recognizing the analogy to previously considered cases and understanding the equivalence of the objects involved.

### What is the significance of using lowercase letters for certain tensors in the script?

-Lowercase letters are used for certain tensors to avoid potential clashes with other embeddings and to signal that the discussion is about the embedding of the curve within the surface.

### What is the curvature normal tensor mentioned in the script?

-The curvature normal tensor is a tensor that describes the curvature of a curve embedded in a surface. It is not an invariant initially but becomes one after index contraction.

### How does the script handle the product rule in the differentiation process?

-The script deliberately avoids using the product rule initially to avoid confusion with the title, but later applies it to derive the relationship among the curvatures.

### What is the relationship between the shift tensor and the curvature normal tensor as described in the script?

-The script describes a second-order relationship where the curvature normal tensor of the embedding is equal to the product of the shift tensor and the shift tensor contracted with the original tensors, plus additional terms from the chain rule.

### What is the geometric meaning of the final answer derived in the script?

-The final answer derived in the script represents a general second-order relationship that holds the key to concepts like principal curvature, and it is a step towards understanding the invariants of the embedded manifolds.

### How does the script address the dimensionality of the embeddings?

-The script emphasizes that the derived relationship is very general and works in full generality with respect to the dimensionalities of the embeddings, applicable even in n-dimensional spaces.

### What are the next steps suggested by the script for further exploration?

-The script suggests that the next steps will involve contracting the Greek indices to work towards invariants, particularly in the context of simplifying assumptions about hypersurfaces, to quickly reach the concept of principal curvatures.

###### Outlines

##### π Exploring Curvature Relationships in Tensor Calculus

This paragraph delves into the intricate relationship among the curvatures derived from the double embedding scenario. The speaker explains the necessity of using the covariant derivative operator to explore these relationships further, as it leads to tensors and invariants. The process involves applying this derivative to both sides of the given equations, taking care to apply the chain rule and considering the shift tensor with respect to the curve. The speaker emphasizes the unfamiliarity of the symbols but encourages viewers to recognize the analogy to previously covered cases. The main focus is on the curvature normal tensor, which is not yet an invariant, and the speaker invites the audience to ponder its properties before revealing the next steps in the calculation.

##### π Applying the Chain Rule to Tensors in Ambient Space

The second paragraph continues the exploration of tensor calculus by applying the chain rule to the shift tensor within the ambient space of the curve embedded in the surface. The speaker clarifies that the chain rule is applicable despite the presence of an additional ambient index. The result of this application is the derivation of the curvature normal tensor, which is initially not an invariant but becomes one after index contraction. The speaker also discusses the use of lowercase letters to distinguish between different embeddings and to avoid potential symbol clashes. The paragraph concludes with the final expression for the curvature normal tensor of the embedding, highlighting the generality of the relationship irrespective of the dimensions of the embeddings.

##### π Unveiling the General Second-Order Relationship and its Implications

The final paragraph presents the second-order relationship between the curvatures, which is a key to understanding principal curvatures and other related concepts. The speaker emphasizes the generality of this relationship, applicable to any dimensionality of embeddings. It is noted that this relationship will have many special cases, especially when dealing with hypersurfaces. The speaker also mentions the upcoming focus on hypersurfaces and the process of contracting Greek indices to work towards invariants. The goal is to simplify the discussion and quickly arrive at the concept of principal curvatures, which will be the main topic of the next video.

###### Mindmap

###### Keywords

##### π‘Shift Tensors

##### π‘Curvatures

##### π‘Covariant Derivative

##### π‘Manifolds

##### π‘Chain Rule

##### π‘Curvature Normal Tensor

##### π‘Invariants

##### π‘Principal Curvatures

##### π‘Embedding

##### π‘Hypersurfaces

###### Highlights

Introduction to the relationship among the curvatures obtained from the shift tensors by differentiation.

Use of covariant derivative as the proper operator to apply on the surface to lead to tensors and invariants.

Differentiation of the shift tensor with the capital Greek index to find the relationship among the curvatures.

Explanation of the equivalence and analogy to previously considered cases with three different manifolds.

Application of the covariant derivative to both sides of the equations involving the shift tensor.

Discussion on the curvature normal tensor and its properties before becoming an invariant.

The product rule's application and its implications in tensor calculus.

Introduction of the chain rule in the context of the shift tensor with respect to the derivative on the curve.

Explanation of the shift tensor's definition in the entire ambient space with respect to the curve.

Derivation of the curvature normal tensor using the chain rule and its geometric implications.

Different notation used for the embedding of the curve within the surface to avoid symbol clashes.

Final expression of the second-order relationship between the curvature normal tensors.

Generality of the relationship with respect to the dimensionalities of the embeddings.

Anticipation of the concept of principal curvature and its exploration in the next video.

Importance of contracting the Greek indices to work towards invariants in the context of hypersurfaces.

The video's goal to simplify the discussion by focusing on hypersurfaces and quickly reaching the concept of principal curvatures.

###### Transcripts

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