func init() {
prime := struct{}{}
primes = make(map[int]struct{})
for _, p := range utils.PrimesUpTo(10000) {
primes[int(p)] = prime
}
}
func main() {
// From wiki:
// A fraction in lowest terms with a prime denominator other than 2 or 5 (i.e. coprime to 10) always produces a repeating decimal.
// The period of the repeating decimal of 1/p is equal to the order of 10 modulo p
var d uint64
var maxPeriod int = 0
// skipping 2, 3, 5
for _, p := range utils.PrimesUpTo(1000)[3:] {
period := order(10, p)
if period > maxPeriod {
maxPeriod, d = period, p
}
}
fmt.Println(d)
}
func init() {
// Can skip 8/9 digit numbers as
// 1 + .. + 9 = 45 => div by 3
// 1 + .. + 8 = 35 => div by 3
primes = utils.PrimesUpTo(7654321)
}
func init() {
primes = utils.PrimesUpTo(15000)
}
The number 3797 has an interesting property. Being prime itself, it is possible to continuously remove digits from left to right, and remain prime at each stage: 3797, 797, 97, and 7. Similarly we can work from right to left: 3797, 379, 37, and 3.
Find the sum of the only eleven primes that are both truncatable from left to right and right to left.
NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.
*/
package main
import (
"fmt"
"strconv"
"utils"
)
var primes []uint64 = utils.PrimesUpTo(1000000)
var present struct{} = struct{}{}
var primesMap map[uint64]struct{} = map[uint64]struct{}{
2: present,
3: present,
5: present,
7: present,
}
var truncatable map[uint64]struct{} = make(map[uint64]struct{})
func main() {
// Start at the first prime > 10
for i := 4; i < len(primes); i++ {
p := primes[i]
primesMap[p] = present