- Label propagation
- Language identification
- Language modeling
- Language translation
- Large margin classifiers
- Latent Dirichlet allocation
- Latent semantic analysis
- Layer-wise relevance propagation
- Learning from imbalanced datasets
- Learning from noisy data
- Learning to rank
- Lexical analysis
- Linear algebra
- Linear discriminant analysis
- Linear dynamical systems
- Linear programming
- Linear regression
- Linear-quadratic-Gaussian control
- Link analysis
- Link prediction
- Local binary patterns
- Local feature extraction
- Locality-sensitive hashing
- Logical inference
- Logical reasoning
- Logistic regression
- Long short-term memory networks
- Low-rank matrix completion
- Low-rank matrix factorization

# What is Low-rank matrix completion

##### Low-rank Matrix Completion: A Powerful Tool to Recover Missing Data

Low-rank matrix completion is an essential technique in data analysis that involves the recovery of missing data in a low-dimensional matrix. The technique involves the identification of the matrix's underlying low-rank structure and then making use of this to approximate the matrix's missing values. It is a popular method in many scientific applications and has proved to be very effective in the recovery of corrupted or missing data.

##### What is Low-rank Matrix Completion?

A low-rank matrix is a matrix that can be approximated with fewer rank matrices than the original matrix. A matrix's rank refers to the maximum number of linearly independent rows or columns in a matrix. Low-rank matrix completion involves finding a low-rank approximation of a matrix with missing entries. This technique uses the matrix's low-rank structure to approximate its missing values. This makes low-rank matrix completion an essential technique in the recovery of data from incomplete or corrupted data.

##### The Application of Low-rank Matrix Completion

Low-rank matrix completion is applied in various fields, including computer vision, biology, and finance. One popular application of low-rank matrix completion is in the recovery of images corrupted by noise or incomplete observations. The technique is also used in the prediction of unknown biological interactions in drug discovery and has proved to be essential in the detection of anomalies in financial data.

##### The Mathematical Formulation of Low-rank Matrix Completion

The mathematical formulation of low-rank matrix completion involves the estimation of missing data in a matrix, given partial observations of that matrix. Let the matrix we want to complete be represented as M, and let the observed entries be represented as M_o. The task of low-rank matrix completion is to estimate the missing values of M, given M_o. Mathematically, the problem can be expressed as:

- Minimize ||M||_*
- s.t. M_o = P_omega(M) + E

where ||M||_* is the nuclear norm, P_omega(M) denotes the observed entries of M, and E represents the error in the estimation of missing values. The nuclear norm of a matrix is the sum of the singular values of the matrix. The minimization of the nuclear norm is equivalent to finding an approximation of the rank of the matrix.

##### Advantages of Low-rank Matrix Completion

Low-rank matrix completion has several advantages that make it a powerful technique in the recovery of missing or corrupted data. Firstly, the technique is effective in handling missing data that cannot be imputed using traditional methods such as mean or median imputation. Secondly, the technique is less prone to overfitting than more complex imputation methods. Thirdly, low-rank matrix completion can be used to estimate missing data in real-time because it is computationally efficient.

##### Challenges of Low-rank Matrix Completion

Low-rank matrix completion also has several challenges that make it a complex technique. Firstly, the technique is sensitive to noise in the input data. This means that the quality of the recovered data depends on the quality of the observed data. Secondly, the technique's accuracy depends on the rank of the matrix, which is often unknown a priori. Thirdly, the technique's computational complexity increases with the size of the input matrix, making it challenging to apply in large-scale data analysis. Finally, the technique often requires the selection of several parameters, which can affect the quality of the recovered data.

##### Conclusion

Low-rank matrix completion is an essential technique in data analysis that involves the recovery of missing data in a low-dimensional matrix. The technique has many applications in various scientific fields and has proved to be very effective in the recovery of corrupted or missing data. Low-rank matrix completion is computationally efficient and can be used to estimate missing data in real-time. However, the technique is sensitive to noise in the input data, and its accuracy depends on the rank of the matrix, which is often unknown a priori. Despite these challenges, low-rank matrix completion is a powerful technique in data analysis and is an essential tool in the recovery of missing or corrupted data.