convenience for the authors consulting literatures. To these persons and units, here
the authors are pleased to express their gratitude. Meanwhile, thank Springer Verlag
for publishing this book.
Many scholars all think that the subject for the calculus of variations is too
wonderful for words, that this is indeed the case, so the authors have been studying
it for a long time. But the authors think that the calculus of variations is more like
Lao Dan’s one word in the Daode Jing “Reaching from the mystery into the deeper
mystery is the gate to the secret of all life”. Seeing this book is to master the key to
open the gate to the secret of all life about the calculus of variations, at any time a
person can open and enter the gate to the secret of all life, and experience the
endless wit and interest from it.
The Vajracchedika-prajna-paramita Sutra well said: “When the Tathagata
expounds the Dharma, there is really no Dharma to teach: but this is expediently
called teaching the Dharma.” It is like this for present book.
Beijing, China
Dazhong Lao
Dandong, China
May 2019
Shanshan Zhao
Preface
ix