selecting objects where some of them are identical
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From: emekadavid on 5 Jul 2010 10:46 I have a problem with some selection or combination. I know that r permutation of n objects where some of the objects are identical is nPr divided by factorials of the number of identical objects. also that r combination or selection of n objects is nPr divided by r factorial. But how does one do r combination or selection of n objects when some of the objects are identical? this is a home assignment. TIA xnt
From: Arturo Magidin on 5 Jul 2010 13:12 On Jul 5, 9:46 am, emekadavid <nnaemeka.da...(a)gmail.com> wrote: > I have a problem with some selection or combination. > I know that r permutation of n objects where some of the objects are > identical is nPr divided by factorials of the number of identical > objects. > also that r combination or selection of n objects is nPr divided by r > factorial. > But how does one do r combination or selection of n objects when some > of > the objects are identical? You can always do it by cases: first select how many of the identical objects you are selecting, then pick the rest; add over all possible choices. -- Arturo Magidin
From: emekadavid on 16 Jul 2010 12:39
On Jul 5, 6:12 pm, Arturo Magidin <magi...(a)member.ams.org> wrote: > On Jul 5, 9:46 am, emekadavid <nnaemeka.da...(a)gmail.com> wrote: > > > I have a problem with some selection or combination. > > I know that r permutation of nobjectswhere some of theobjectsare > > identical is nPr divided by factorials of the number of identical > >objects. > > also that r combination or selection of nobjectsis nPr divided by r > > factorial. > > But how does one do r combination or selection of nobjectswhen some > > of > > theobjectsare identical? > > You can always do it by cases: first select how many of the identicalobjectsyou areselecting, then pick the rest; add over all possible > choices. > > -- > Arturo Magidin thanks. i have to add right?thought it was something similar to objects in a circle. thanks |