Lovelock, D. (1969). "The uniqueness of the Einstein field equations in a four-dimensional space". Archive for Rational Mechanics and Analysis. 33 (1): 54–70. Bibcode:1969ArRMA..33...54L. doi:10.1007/BF00248156.
Lovelock, D. (1969). "The uniqueness of the Einstein field equations in a four-dimensional space". Archive for Rational Mechanics and Analysis. 33 (1): 54–70. Bibcode:1969ArRMA..33...54L. doi:10.1007/BF00248156.