an inverstion is i<j and a[i]>a[j] then (a[i],a[j]) is called an inverstion
take the list 4,3,2,1
inverstions:
(4,1)(3,1)(2,1)
(4,2)(3,2)
(4,3)
= 3+2+1= 6
its nothing but sigma(4-1)
if there is a list of 11 elements in decreasing order, then total inverstions = sigma(11-1) = 10*11/2 = 55