Theory of Machine
The relation between number of links (l) and number of joints (j) in a kinematic chain is

l = (3/4).(j + 3)
l = j + 4
l = (1/2).(j + 2)
l = (2/3).(j + 2)

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Theory of Machine
Lower pairs are those which have

Point or line contact between the two elements when in motion
Surface contact between the two elements when in motion
Two elements that permit relative motion
Elements of pairs not held together mechanically

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