# spectral-norm Python 3 #7 program

## source code

```# The Computer Language Benchmarks Game
# https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
#
# Contributed by Sebastien Loisel
# Fixed by Isaac Gouy
# Sped up by Josh Goldfoot
# Dirtily sped up by Simon Descarpentries
# Used list comprehension by Vadim Zelenin
# Concurrency by Jason Stitt
# Concurrency simplified by Matt Vollrath

from itertools import repeat
from math import sqrt
from multiprocessing import Pool
from sys import argv

def eval_A(i, j):
ij = i + j
return 1.0 / (ij * (ij + 1) / 2 + i + 1)

def A_sum(u, i):
x = 0
for j, u_j in enumerate(u):
x += eval_A(i, j) * u_j
return x

def At_sum(u, i):
x = 0
for j, u_j in enumerate(u):
x += eval_A(j, i) * u_j
return x

def multiply_AtAv(u):
r = range(len(u))

tmp = pool.starmap(
A_sum,
zip(repeat(u), r)
)
return pool.starmap(
At_sum,
zip(repeat(tmp), r)
)

def main():
n = int(argv[1])
u = [1] * n

for _ in range(10):
v = multiply_AtAv(u)
u = multiply_AtAv(v)

vBv = vv = 0

for ue, ve in zip(u, v):
vBv += ue * ve
vv  += ve * ve

result = sqrt(vBv/vv)
print("{0:.9f}".format(result))

if __name__ == '__main__':
with Pool(processes=4) as pool:
main()
```

## notes, command-line, and program output

```NOTES:
Python 3.12.3

Tue, 28 May 2024 01:38:55 GMT

MAKE:
mv spectralnorm.python3-7.python3 spectralnorm.python3-7.py
pyright .
/home/dunham/all-benchmarksgame/benchmarksgame_i53330/spectralnorm/tmp/spectralnorm.python3-7.py
/home/dunham/all-benchmarksgame/benchmarksgame_i53330/spectralnorm/tmp/spectralnorm.python3-7.py:61:26 - error: "v" is possibly unbound (reportPossiblyUnboundVariable)
1 error, 0 warnings, 0 informations
make: [/home/dunham/all-benchmarksgame/2000-benchmarksgame/nanobench/makefiles/u64q.programs.Makefile:421: spectralnorm.python3-7.python3_run] Error 1 (ignored)

4.36s to complete and log all make actions

COMMAND LINE:
/opt/src/Python-3.12.3/bin/python3 -OO spectralnorm.python3-7.py 5500

PROGRAM OUTPUT:
1.274224153
```