Question on “smart” replacing in mathematica

Question

How do I tell mathematica to do this replacement smartly? (or how do I get smarter at telling mathematica to do what i want)

expr = b + c d + ec + 2 a;
expr          


        

Answer 1

You can use a customised version of FullSimplify by supplying your own transformations to FullSimplify and let it figure out the details:

In[1]:= MySimplify[expr_,equivs_]:= FullSimplify[expr,
          TransformationFunctions ->
            Prepend[
              Function[x,x-#]&/@Flatten@Map[{#,-#}&,equivs/.Equal->Subtract],
              Automatic
            ]
        ]
In[2]:= MySimplify[2a+b+c*d+e*c, {a+b==1}]
Out[2]= a + c(d + e) + 1

equivs/.Equal->Subtract turns given equations into expressions equal to zero (e.g. a+b==1 -> a+b-1). Flatten@Map[{#,-#}&, ] then constructs also negated versions and flattens them into a single list. Function[x,x-#]& /@ turns the zero expressions into functions, which subtract the zero expressions (the #) from what is later given to them (x) by FullSimplify.

It may be necessary to specify your own ComplexityFunction for FullSimplify, too, if your idea of simple differs from FullSimplify's default ComplexityFunction (which is roughly equivalent to LeafCount), e.g.:

MySimplify[expr_, equivs_] := FullSimplify[expr,
  TransformationFunctions ->
    Prepend[
      Function[x,x-#]&/@Flatten@Map[{#,-#}&,equivs/.Equal->Subtract],
      Automatic
    ],
  ComplexityFunction -> (
    1000 LeafCount[#] + 
    Composition[
      Total,Flatten,Map[ArrayDepth[#]#&,#]&,CoefficientArrays
    ][#] &
  )
]

In your example case, the default ComplexityFunction works fine, though.

Answer 2

For the first case, you might consider:

expr = b + c d + ec + 2 a

PolynomialReduce[expr, {a + b - 1}, {b, a}][[2]]

---

For the second case, consider:

expr = b + c (1 - a) + (-1 + b) (a - 1) + (1 - a - b) d + 2 a;

PolynomialReduce[expr, {x + b - 1}][[2]]

(% /. x -> 1 - b) == expr // Simplify

and:

PolynomialReduce[expr, {a + b - 1}][[2]]

Simplify[% == expr /. a -> 1 - b]