next | previous | forward | backward | up | top | index | toc | Macaulay2 web site
Macaulay2Doc :: lift

lift -- lift to another ring

Synopsis

Description

(Disambiguation: for division of matrices, which is thought of as lifting one homomorphism over another, see instead Matrix // Matrix. For lifting a map between modules to a map between their free resolutions, see extend.)

The ring R should be one of the base rings associated with the ring of f. An error is raised if f cannot be lifted to R.

The first example is lifting from the fraction field of R to R.

i1 : lift(4/2,ZZ)

o1 = 2
i2 : R = ZZ[x];
i3 : f = ((x+1)^3*(x+4))/((x+4)*(x+1))

      2
o3 = x  + 2x + 1

o3 : frac R
i4 : lift(f,R)

      2
o4 = x  + 2x + 1

o4 : R

Another use of lift is to take polynomials in a quotient ring and lift them to the polynomial ring.

i5 : A = QQ[a..d];
i6 : B = A/(a^2-b,c^2-d-a-3);
i7 : f = c^5

                 2
o7 = 2a*c*d + c*d  + 6a*c + b*c + 6c*d + 9c

o7 : B
i8 : lift(f,A)

                 2
o8 = 2a*c*d + c*d  + 6a*c + b*c + 6c*d + 9c

o8 : A
i9 : jf = jacobian ideal f

o9 = {1} | 2cd+6c           |
     {1} | c                |
     {1} | 2ad+d2+6a+b+6d+9 |
     {1} | 2ac+2cd+6c       |

             4       1
o9 : Matrix B  <--- B
i10 : lift(jf,A)

o10 = {1} | 2cd+6c           |
      {1} | c                |
      {1} | 2ad+d2+6a+b+6d+9 |
      {1} | 2ac+2cd+6c       |

              4       1
o10 : Matrix A  <--- A

Elements may be lifted to any base ring, if such a lift exists.

i11 : use B;
i12 : g = (a^2+2*a-3)-(a+1)^2

o12 = -4

o12 : B
i13 : lift(g,A)

o13 = -4

o13 : A
i14 : lift(g,QQ)

o14 = -4

o14 : QQ
i15 : lift(lift(g,QQ),ZZ)

o15 = -4

The functions lift and substitute are useful to move numbers from one kind of coefficient ring to another.

i16 : lift(3.0,ZZ)

o16 = 3
i17 : lift(3.0,QQ)

o17 = 3

o17 : QQ

A continued fraction method is used to lift a real number to a rational number, whereas promote uses the internal binary representation.

i18 : lift(123/2341.,QQ)

       123
o18 = ----
      2341

o18 : QQ
i19 : promote(123/2341.,QQ)

       7572049608428139
o19 = ------------------
      144115188075855872

o19 : QQ
i20 : factor oo

      3*811*39877*78045679
o20 = --------------------
                57
               2

o20 : Expression of class Divide

For numbers and ring elements, an alternate syntax with ^ is available, analogous to the use of _ for promote.

i21 : .0001^QQ

        1
o21 = -----
      10000

o21 : QQ
i22 : .0001_QQ

        7378697629483821
o22 = --------------------
      73786976294838206464

o22 : QQ

See also

Ways to use lift :