Nagarjuna, also known as the "second Buddha," was a prominent Buddhist philosopher and founder of the Madhyamaka school of thought. He is considered one of the most influential figures in Mahayana Buddhism and his teachings have had a profound impact on the development of Buddhist philosophy.

Nagarjuna's key texts include the Mulamadhyamakakarika, also known as the Fundamental Verses on the Middle Way, and the Vigrahavyavartani, also known as the Treatise on the Refutation of the Objections. These two texts are considered the most important works of Nagarjuna and are widely studied and revered by Buddhist scholars and practitioners.

The Mulamadhyamakakarika is a collection of 27 chapters, each containing 25 verses, that expounds on the concept of shunyata or emptiness. In this text, Nagarjuna argues that all phenomena are empty of inherent existence and that the true nature of reality is beyond conceptualization. He uses logical reasoning and dialectics to deconstruct the concept of inherent existence and to show that all things are interdependent and lack inherent existence.

The Vigrahavyavartani is a commentary on the Mulamadhyamakakarika and further elaborates on the concept of emptiness. In this text, Nagarjuna responds to objections and criticisms of his teachings and defends the Madhyamaka school against other Buddhist schools and non-Buddhist philosophical systems. He also delves deeper into the concept of emptiness and its implications for understanding the nature of reality.

Apart from these two major works, Nagarjuna also wrote other important texts such as the Ratnavali, which discusses the Bodhisattva path, and the Suhrllekha, a letter to a friend that expounds on the concept of emptiness and its practical application in daily life.

Nagarjuna's writings are known for their clarity and logical precision, and his use of dialectics and reasoning has greatly influenced the development of Buddhist philosophy. His teachings on emptiness and the Middle Way have also had a profound impact on other schools of Buddhism, such as Tibetan Buddhism and Zen Buddhism.

In conclusion, Nagarjuna's key texts, the Mulamadhyamakakarika and the Vigrahavyavartani, are essential for understanding the Madhyamaka school of thought and the concept of emptiness in Buddhism. His writings continue to be studied and debated by scholars and practitioners, making him one of the most influential figures in Buddhist philosophy.