diff --git a/project_euler/problem_23/sol1.py b/project_euler/problem_23/sol1.py new file mode 100644 index 000000000..e76be0530 --- /dev/null +++ b/project_euler/problem_23/sol1.py @@ -0,0 +1,51 @@ +""" +A perfect number is a number for which the sum of its proper divisors is exactly +equal to the number. For example, the sum of the proper divisors of 28 would be +1 + 2 + 4 + 7 + 14 = 28, which means that 28 is a perfect number. + +A number n is called deficient if the sum of its proper divisors is less than n +and it is called abundant if this sum exceeds n. + +As 12 is the smallest abundant number, 1 + 2 + 3 + 4 + 6 = 16, the smallest +number that can be written as the sum of two abundant numbers is 24. By +mathematical analysis, it can be shown that all integers greater than 28123 +can be written as the sum of two abundant numbers. However, this upper limit +cannot be reduced any further by analysis even though it is known that the +greatest number that cannot be expressed as the sum of two abundant numbers +is less than this limit. + +Find the sum of all the positive integers which cannot be written as the sum +of two abundant numbers. +""" + +def solution(limit = 28123): + """ + Finds the sum of all the positive integers which cannot be written as + the sum of two abundant numbers + as described by the statement above. + + >>> solution() + 4179871 + """ + sumDivs = [1] * (limit + 1) + + for i in range(2, int(limit ** 0.5) + 1): + sumDivs[i * i] += i + for k in range(i + 1, limit // i + 1): + sumDivs[k * i] += k + i + + abundants = set() + res = 0 + + for n in range(1, limit + 1): + if sumDivs[n] > n: + abundants.add(n) + + if not any((n - a in abundants) for a in abundants): + res+=n + + return res + + +if __name__ == "__main__": + print(solution())