{{{id=1|
def prob_two(n):
    '''Probability of *at least* two birthdays in a group of n people'''
    all_different = prod(srange(365-n+1, 366))
    return N(1 - all_different/(365^n), digits = 10)
///
}}}

<p>50% probability at n=23 people.</p>

{{{id=2|
prob_two(23)
///
}}}

<p>90% at n=41 people</p>

{{{id=3|
prob_two(41)
///
}}}

{{{id=5|
data = [(i, prob_two(i)) for i in range(1, 40)]
list_plot(data)
///
}}}