From 1eefd58ca4fdb5d2f51f657bfd70c9a89a4707db Mon Sep 17 00:00:00 2001
From: Iru Cai <mytbk920423@gmail.com>
Date: Thu, 24 May 2018 21:39:58 +0800
Subject: initial commit

---
 euler21.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)
 create mode 100644 euler21.c

(limited to 'euler21.c')

diff --git a/euler21.c b/euler21.c
new file mode 100644
index 0000000..d18e556
--- /dev/null
+++ b/euler21.c
@@ -0,0 +1,52 @@
+#include <stdio.h>
+#include <math.h>
+#define MAX 9999
+
+int firstdiv(int n)
+{
+	int i;
+	int q=ceil(sqrt(n));
+	for (i=2;i<=q;++i){
+		if (n%i==0)
+			return i;
+	}
+	return n;
+}
+
+int divexp(int n,int d)
+{
+	int dd=1;
+	while (n%d==0){
+		n/=d;
+		dd*=d;
+	}
+	return dd;
+}
+
+int sumdiv(int n)
+{
+	if (n==1)
+		return 1;
+
+	int fd=firstdiv(n);
+	int fdexp=divexp(n,fd);
+	return sumdiv(n/fdexp)*(fdexp*fd-1)/(fd-1);
+}
+
+int main()
+{
+	int d[MAX+1],i,sum=0;
+	for (i=1;i<=MAX;++i){
+		d[i]=sumdiv(i)-i;
+	}
+	for (i=1;i<=MAX;++i){
+		if (i<d[i] && d[i]<=MAX && i==d[d[i]]){
+			sum+=i+d[i];
+		}else if (d[i]>MAX && sumdiv(d[i])-d[i]==i){
+			sum+=i;
+		}
+	}
+	printf("%d\n",sum);
+	return 0;
+}
+	
-- 
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