Permutations[{i,j,k}]

Flatten[%]

Position[%,i]

NestList[ Cos, x, 4 ]

Plot[ Evaluate[%], {x,0,Pi} ]

?NestList

Map[ f, {a,b,c,d}]

Map[ Function[x, 1+x^2], {a,b,c,d}]

Hilbert[n_] := Table[ 1/(i+j-1), {i,n},{j,n} ]

CharPoly[m_,x_] := Det[m-x IdentityMatrix[Length[m]]] /;
                   MatrixQ[m]

?/;

?MatrixQ

?Mean

Mean[list_List] := Apply[Plus, list] / Length[list]

Variance[list_List] := Mean[ (list - Mean[list])^2 ]

Mean[{a1,a2,a3,a4,a5}]

N[ Sqrt[ Variance[{1,2,3,4,5,6,7,8,9,10}] ] ]

RandomWalk[n_Integer] :=
      FoldList[Plus,0,Table[Random[] - 1/2, {n}]]

RandomWalk[1000] ;

ListPlot[%, PlotJoined -> True ]