## Developer Reference

• 0.9
• 09/09/2020
• Public Content
Contents

# The FEAST Algorithm

The Extended Eigensolver functionality is a set of high-performance numerical routines for solving symmetric standard eigenvalue problems,
A
x
=
λ
x
, or generalized symmetric-definite eigenvalue problems,
A
x
=
λ
B
x
. It yields all the eigenvalues (
λ
) and eigenvectors (
x
) within a given search interval
[
λ
min
,
λ
max
]
. It is based on the FEAST algorithm, an innovative fast and stable numerical algorithm presented in [Polizzi09], which fundamentally differs from the traditional Krylov subspace iteration based techniques (Arnoldi and Lanczos algorithms [Bai00]) or other Davidson-Jacobi techniques [Sleijpen96]. The FEAST algorithm is inspired by the density-matrix representation and contour integration techniques in quantum mechanics.
The FEAST numerical algorithm obtains eigenpair solutions using a numerically efficient contour integration technique. The main computational tasks in the FEAST algorithm consist of solving a few independent linear systems along the contour and solving a reduced eigenvalue problem. Consider a circle centered in the middle of the search interval
[
λ
min
,
λ
max
]
. The numerical integration over the circle in the current version of FEAST is performed using
N
e
x
e
the
e
-th
Gauss node associated with the weight
ω
e
. For example, for the case
N
e
= 8
:

(
x
1
,
ω
1
) = (0.183434642495649 , 0.362683783378361),

(
x
2
,
ω
2
) = (-0.183434642495649 , 0.362683783378361),

(
x
3
,
ω
3
) = (0.525532409916328 , 0.313706645877887),

(
x
4
,
ω
4
) = (-0.525532409916328 , 0.313706645877887),

(
x
5
,
ω
5
) = (0.796666477413626 , 0.222381034453374),

(
x
6
,
ω
6
) = (-0.796666477413626 , 0.222381034453374),

(
x
7
,
ω
7
) = (0.960289856497536 , 0.101228536290376), and

(
x
8
,
ω
8
) = (-0.960289856497536 , 0.101228536290376).

The figure FEAST Pseudocode shows the basic pseudocode for the FEAST algorithm for the case of real symmetric (left pane) and complex Hermitian (right pane) generalized eigenvalue problems, using
N
for the size of the system and
M
for the number of eigenvalues in the search interval (see [Polizzi09]).
The pseudocode presents a simplified version of the actual algorithm. Refer to http://arxiv.org/abs/1302.0432 for an in-depth presentation and mathematical proof of convergence of FEAST.
 A: real symmetric B: symmetric positive definite (SPD) ℜ{x}: real part of x A: complex Hermitian B: Hermitian positive definite (HPD)  #### Product and Performance Information

1

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Notice revision #20110804