↑(en) Albert Grünwedel, Agnes C. Gibson, James Burgess, Buddhist art in India. Published by Bernard Quaritch, 1901, page 67: "The wheel (dharmachakra), as already mentioned, was adopted by Buddha's disciples as the symbol of his doctrine, and combined with other symbols - a trident placed above it, etc. - stands for him on the sculptures of the Asoka period."