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# ctop(AbstractSheaf) -- the top Chern class of an abstract sheaf

- Function: ctop
- Usage:
    `ctop F`
- Inputs:
  - F, an abstract sheaf,
- Outputs:
  - a ring element, the top Chern class of $F$

## Description

Here we compute the top Chern class of a vector bundle of rank 6 on the way toward getting the number of lines on quintic threefold.

    i1 : G = flagBundle{3,2}

    o1 = G

    o1 : a flag bundle with subquotient ranks {3, 2}

    i2 : B = symmetricPower_5 last bundles G

    o2 = B

    o2 : an abstract sheaf of rank 6 on G

    i3 : ctop B

              3
    o3 = 2875H
              2,2

                                                QQ[][H   ..H   , H   ..H   ]
                                                      1,1   1,3   2,1   2,2
    o3 : ---------------------------------------------------------------------------------------------------------
         (- H    - H   , - H    - H   H    - H   , - H    - H   H    - H   H   , - H   H    - H   H   , -H   H   )
             1,1    2,1     1,2    1,1 2,1    2,2     1,3    1,2 2,1    1,1 2,2     1,3 2,1    1,2 2,2    1,3 2,2

    i4 : degree oo

    o4 = {6}

    o4 : List

    i5 : integral ooo

    o5 = 2875

## Ways to use this method:

- ctop(AbstractSheaf) -- the top Chern class of an abstract sheaf

The source of this document is in /build/macaulay2-88fgJW/macaulay2-1.25.11+ds/M2/Macaulay2/packages/Schubert2.m2:1857:0.