Given a procedure/function $Select(S,r)$ - which selects element of rank r from set Swhich uses at most $|S|. constant$ comparisonsWe design another function $Multiselect(S,R)$with $R =\{r_1<r_2<...<r_k\}$returns $X =\{x_1<x_2<...<x_k\}$  such that rank of $x_i$ is $r_i$What is the minimum no of comparisons for this function?From Wikipedia I found the upper bound for it -But while utilising this for deriving the answer I got it totally messed up!I tried using algorithm to utilise Select(S,r) repeatedly each timeeach time decreasing the search space by logn  ... but I guess the answer could be foundin a more concise mathematical way.By the way, answer given was - $constant.|S|(1+logR)$  
Selection of K elements of given ranks
complexity theory;algorithm analysis
Hint: Find first the element of rank $r_{(k+1)/2}$, and split the list accordingly into two parts $S_1,S_2$ satisfying $|S_1| + |S_2| = |S|$. Now repeat.