Saltire detected the symbol, which often appears with balls in the quadrants formed by the arms of the chi-cross, in standards that appear on the coins of Persepolis. She suggested that early Christians endorsed its solar symbolism as appropriate to Christ. She also wrote: "although it cannot be proved, ... in the white saltire of St. Andrew we still have a reminiscence of the old standard of the Persepolitan kingdom". In the old European Union standard, a black saltire set in an orange square is the hazard symbol for irritants (Xi) or harmful chemicals (Xn). It indicates a hazard less severe
Symbol of a differential operator terms of a basis of frames "e", "f" of "E" and "F", respectively, the differential operator "P" decomposes into components on each section "u" of "E". Here "P" is the scalar differential operator defined by With this trivialization, the principal symbol can now be written In the cotangent space over a fixed point "x" of "X", the symbol formula_13 defines a homogeneous polynomial of degree "k" in formula_14 with values in formula_15. The differential operator formula_16 is elliptic if its symbol is invertible; that is for each nonzero formula_17 the bundle map formula_18 is invertible. On a compact manifold, it