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#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#define MAXN 1000000000000
#define NRUMAX 10000000
long long ru[NRUMAX];
int nru = 0;
int comp(const void *a, const void *b)
{
long long x = *(long long*)a;
long long y = *(long long*)b;
if (x < y)
return -1;
else if (x > y)
return 1;
else
return 0;
}
int main()
{
long long sum_repunit = 1; /* 1 is a strong repunit */
for (int base = 2; base < 1000000; base++) {
long long repunit = 1 + base;
long long nb = base;
while (1) {
nb *= base;
repunit += nb;
if (repunit >= MAXN)
break;
assert(nru < NRUMAX);
ru[nru] = repunit;
nru++;
}
}
qsort(ru, nru, sizeof(long long), comp);
printf("Find %d strong repunits.\n", nru + 1);
for (int i = 0; i < nru; i++) {
if (i > 0 && ru[i] == ru[i-1])
continue;
sum_repunit += ru[i];
}
printf("Their sum is %lld\n", sum_repunit);
}
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