/usr/share/doc/plotutils/ode/qcd.ode is in plotutils 2.6-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | # You may run this example by doing
#
# ode < qcd.ode | graph -T X -C
#
# This simulates a coupled system of ordinary differential equations that
# arises in the mathematical theory of quantum chromodynamics (QCD).
#
# Source: Griffiths et al., J. Math. Phys.
f' = fp
fp' = -f*g*g
g' = gp
gp' = g*f*f
f = 0
fp = -1
g = 1
gp = -1
print t, f
step 0, 5
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