[eigen] Thank you for creating Eigen!

[Alexander Voigt <alexander.voigt@xxxxxxxxxxxxxxxxxxxxx>]

Dear Eigen developers,

herewith I'd like to thank you very much creating the incredibly fast
and versatile Eigen package!

I am a physicist working in theoretical particle physics and I'm using
Eigen for one of my projects: FlexibleSUSY [1].  FlexibleSUSY is a
spectrum generator, a program that calculates the masses of elementary
particles in a very wide class of particle physics models.

For calculating the particle masses with high precision, one challenge
is to being able to calculate extremely complex matrix expressions
very efficiently (the so-called beta-functions [2]).  These matrix
expressions are so complicated that they usually span multiple pages
and they need to be evaluated tens of thousands of times.  Using
Eigen, we are able to evaluate these complicated expressions in an
incredibly short time, which gives us a big speed improvement over
other spectrum generators (SARAH/SPheno [3], SOFTSUSY [4]) and a high
speed is a premiss to study particle physics models at the LHC
experiment.  Interestingly, the evaluation of the matrix expressions
in C++ using the Eigen library is much faster than the classic matrix
multiplication in Fortran (using Fortran's MatMul() function).

Best regards,
Alexander Voigt

P.S.: One Eigen feature which we're looking forward to is rank-3 and
-4 tensors, which appear in some extended particle physics models.
Thus, we cannot easily handle such models for now due to the missing
support of rank-3 and -4 tensors in Eigen 3.2.

P.P.S.: If you think FlexibleSUSY is relevant enough, I'd be happy if
it could be added to the list of "Projects using Eigen" in the section
"Science".

[1] https://flexiblesusy.hepforge.org
    https://github.com/FlexibleSUSY
    arXiv:1406.2319, arXiv:1609.00371
[2] arxiv:hep-ph/9311340, arxiv:1107.2670, arxiv:1310.7629
[3] http://sarah.hepforge.org/
[4] http://softsusy.hepforge.org/

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Alexander Voigt
Institute for Theoretical Particle Physics and Cosmology
RWTH Aachen
Sommerfeldstr. 14
52074 Aachen
Room: 28A408
Phone: +49 (0)241 80 27049
Fax  : +49 (0)241 80 22187
E-Mail: alexander.voigt@xxxxxxxxxxxxxxxxxxxxx
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