In equations, leave а space before and after arithmetic signs (+, –, *, /, =, , etc.). Omit spaces in sub- and superscripts. a + b,  d < с,  Х = А / В; but Са 2+ ions. If the sign is not part of an equation, it should be closed up to the number. <2, ~5,  –х. Do not leave а space between а function and the parentheses of its argument, but leave spaces between arguments. x(y),  W(x, р), W(x, р, t).   Leave а space (non-breaking space is preferred) between a number and a unit of measure, but do not separate а number and а symbol or symbols. 25 m/s s,  7.9 kW h, but 5xy 2t.  To indicate multiplication, insert a space between metric units , but insert a hyphen in customary units. mD·m, m³/day, kPa,  mD-ft, B/D-psi.