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Maths Permutations & Combination JEE (Main) / AIEEE Problems (Previous Years) Subjective Type
Published on: August 14, 2026

Find the number of functions f : A → B where n(a) = m , n(b) = t , which are non decreasing

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Sol. Let A = {a 1 , a 2 , ..... a m } , B = {b 1 , b 2 , ..... b t } with a 1 > a 2 > ....> a m and b 1 > b 2 > ....... > b t

Now for non decreasing function

f(a 1 ) > f(a 2 ) > ..... > f(a m )

where {f(a 1 ) , f(a 2 ) ...... f(a m )} {b 1 , b 2 , ..... b t }

Let us introduce (m – 1) dummy numbers C 1 , C 2 , .... C m – 1 and add into the set B, and then take m numbers from the new B in ways

it is the required no. of non decreasing function from A → B.

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