What is " N " ?
and to that add the product found by multiplying the frequency for a change of energy from state n - a to state n - b by the frequency for a change of energy from state n - b to state n - c , < br > and so forth : < br > Symbolically that is : < br > f ( n , n - a ) * f ( n - a , n - b )) + < br > f ( n - a , n - b ) * f ( n - b , n - c ) + < br > etc.