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f = lambda x, a, b, c: 15 - exp(a * x**b) + a * c * x**b

xkcd()
figure(figsize=(5, 4))
plot(x, f(x, 1.75, 1.3, 0.95), label='early')
plot(x, f(x, 1.7, 1, 0.5), label='late')
xlim(1, 0)
ylim(10, 15)
xlabel(r'$\log\ (1+z)$')
ylabel(r'$\mathrm{log\ Halo\ Mass}$')
subplots_adjust(bottom=0.2, left=0.2)
legend(title='formation time', frameon=False)