help on calculating sum of combinations

[sanjiv kapur]

hello
i am a newbie for using octave (on rh9.0) ,though used a little matlab 
before.may i request help in solving the problem as under,in case a function 
has been developed by somebody.for a matrix or an array,i want to calculate 
the sum of various combinations, i.e

if A=[2, 3, 4, 5, 6, 7], how do i get

a1=2+3+4+5+6+7
a2=2*3+2*4+2*5+2*6+2*7+3*4+3*5+3*6+3*7+4*5+4*6+4*7+...+6*7
a3=2*3*4+2*3*5+2*3*6+2*3*7+2*4*5+2*4*6+2*4*7+2*5*6...+5*6*7
a4=2*3*4*5+2*3*4*6+2*3*4*7+2*3*5*6+...+4*5*6*7
a5=sum of combinations taking five elements at a time
a6=2*3*4*5*6*7

is there a function to calculate the above.please guide me.
thanks and regards
sanjiv

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