All of these allow creating new extension methods of existing types, albeit by three different methods.

I'm assuming that your "2001.to_year - 1" is "the date time for 2001 minus 1 day"

rust (traits)

    (pow::<BigInt>(2u8, 1000).digits().sum()
    (2..10_000).amicable_numbers().sum()
    1901.to_year().upto(2001.to_year() - 1.to_day()).days().iter().filter(|d| d.mday + d.wday == 1).count()

    sum . digits $ 2 ^ 1000
    sum $ amicable_numbers [2..10000]
    count $ filter (\d -> mday d + wday d == 1) [year 1901 .. year 2000 - day 1]

    BigInteger.Pow(2, 1000).digits().sum()
    Enumerable.range(2,10000).amicable_numbers().sum()
    1901.ToYear().UpTo(2001.ToYear() - 1.ToDay()).Days().Where(d => d.mday + d.wday == 1).Count()

    bigInt(2).pow(1000).digits().sum()
    _.range(2, 10000).amicable_numbers().sum()
    1901.to_year().upto(2001.to_year().minus(1)).days().filter(d => d.mday + d.wday === 1).length()