# Assembling Gram matrix

**URL:** <https://fenicsproject.discourse.group/t/assembling-gram-matrix/15964>\
**Category:** General\
**Created:** [October 3, 2024, 10:29am UTC](https://fenicsproject.discourse.group/t/assembling-gram-matrix/15964 "2024-10-03T10:29:38Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![dajansen9](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dajansen9/32/6962_2.png) [@dajansen9](https://fenicsproject.discourse.group/u/dajansen9)\
**Post date:** [October 3, 2024, 10:29am UTC](https://fenicsproject.discourse.group/t/assembling-gram-matrix/15964/1 "2024-10-03T10:29:38Z")

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Hi all,

This probably comes off as a very basic question, however, I have been searching for a couple of hours and I don’t know how to continue. Hopefully, someone here can explain to me what I am doing wrong! I am trying to compute the Gram matrix of a solution. My MWE looks as follows:

```auto
from mpi4py import MPI
from dolfinx.fem import functionspace,form
from dolfinx.fem.petsc import assemble_matrix
from dolfinx.mesh import create_unit_interval
from ufl import TestFunction, TrialFunction, inner, grad, dx, dot

mesh = create_unit_interval(MPI.COMM_SELF, 8)
V = functionspace(mesh,('CG',1))
u = TrialFunction(V)
v = TestFunction(V)

G1 = form(inner(u,v) * dx)
G1_assembled = assemble_matrix(G1)

print(G1)

```

However, I don’t know how to set this into an array that I can save or pickle. Could someone please explain to me how to continue? Thanks a lot in advance!  
Kind regards, Daan

---

<div class="post-metadata">

**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\
**Post date:** [October 3, 2024, 3:29pm UTC](https://fenicsproject.discourse.group/t/assembling-gram-matrix/15964/2 "2024-10-03T15:29:02Z")

</div>

> [@dajansen9](#):
>
> `G1_assembled = assemble_matrix(G1)`

Just add

```python
G1_assembled.assemble()

```

and then

```python
G1_dense = G1_assembled[:,:]

```

You could also use the built in matrices, and do the following

```python
from mpi4py import MPI
from dolfinx.fem import functionspace,form, assemble_matrix
from dolfinx.mesh import create_unit_interval
from ufl import TestFunction, TrialFunction, inner, dx

mesh = create_unit_interval(MPI.COMM_SELF, 8)
V = functionspace(mesh,('CG',1))
u = TrialFunction(V)
v = TestFunction(V)

G1 = form(inner(u,v) * dx)
G1_assembled = assemble_matrix(G1)
G1_assembled.scatter_reverse()
print(G1_assembled.to_scipy())

```

to get a scipy csr matrix