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 "X", and suppose is a differential operator of order formula_6. In local coordinates on "X", we have where, for each multi-index α, formula_8 is a bundle map, symmetric on the indices α. The "k" order coefficients of "P" transform as a symmetric tensor from the tensor product of the "k" symmetric power of the cotangent bundle of "X" with "E" to "F". This symmetric tensor is known as the principal symbol (or just the symbol) of "P". The coordinate system "x" permits a local trivialization of the cotangent bundle by the coordinate differentials d"x", which determine fiber coordinates ξ. In