Factorial primes: Difference between revisions
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Line 192:
9: 479001600! - 1 = 479001599
10: 87178291200! - 1 = 87178291199
</pre>
=== Another Way with BigIntegers ===
<syntaxhighlight lang=java>
import java.math.BigInteger;
public class MainApp {
public static void main(String[] args) {
//Used to measure total runtime of program.
long starttime = System.nanoTime();
//How many primes found, how many primes wanted, loop counter.
int countOfPrimes = 0;
final int targetCountOfPrimes = 30;
long f = 1;
//Starting BigInteger at 1.
BigInteger biFactorial = BigInteger.ONE;
while (countOfPrimes < targetCountOfPrimes) {
//Each loop, multiply the number by the loop
//counter (f) to increase factorial much more quickly.
biFactorial = biFactorial.multiply(BigInteger.valueOf(f));
// one less than the factorial.
BigInteger biMinusOne = biFactorial.subtract(BigInteger.ONE);
// one more than the factorial.
BigInteger biPlusOne = biFactorial.add(BigInteger.ONE);
//Determine if the numbers are prime with a probability of 100
boolean primeMinus = biMinusOne.isProbablePrime(100);
boolean primePlus = biPlusOne.isProbablePrime(100);
//Make the big number look like a pretty string for output.
String biMinusOneString = convert(biMinusOne);
String biPlusOneString = convert(biPlusOne);
//If the number was prime, output and increment the primt counter.
if (primeMinus) {
countOfPrimes++;
System.out.println(
countOfPrimes + ": " + f + "! - 1 = " + biMinusOneString);
}
if (primePlus && f > 1) {
countOfPrimes++;
System.out.println(countOfPrimes + ": " + f + "! + 1 = " + biPlusOneString);
}
//Increment loop counter.
f++;
}
//Calculate and display program runtime.
long stoptime = System.nanoTime();
long runtime = stoptime - starttime;
System.out.println("Program runtime: " + runtime + " ns (~" + runtime/1_000_000_000 + " seconds)");
}
//Method to make output pretty
private static String convert(BigInteger bi) {
String s = bi.toString();
int l = s.length();
if (l >= 40) {
s = s.substring(0,39);
s += "... : " + l + " digits";
}
return s;
}
}
</syntaxhighlight>
===output===
<pre>
1: 2! + 1 = 3
2: 3! - 1 = 5
3: 3! + 1 = 7
4: 4! - 1 = 23
5: 6! - 1 = 719
6: 7! - 1 = 5039
7: 11! + 1 = 39916801
8: 12! - 1 = 479001599
9: 14! - 1 = 87178291199
10: 27! + 1 = 10888869450418352160768000001
11: 30! - 1 = 265252859812191058636308479999999
12: 32! - 1 = 263130836933693530167218012159999999
13: 33! - 1 = 8683317618811886495518194401279999999
14: 37! + 1 = 137637530912263450463159795815809024000... : 44 digits
15: 38! - 1 = 523022617466601111760007224100074291199... : 45 digits
16: 41! + 1 = 334525266131638071081700620534407516651... : 50 digits
17: 73! + 1 = 447011546151268434089125713812505111007... : 106 digits
18: 77! + 1 = 145183092028285869634070784086308284983... : 114 digits
19: 94! - 1 = 108736615665674308027365285256786601004... : 147 digits
20: 116! + 1 = 339310868445189820119825609358857320323... : 191 digits
21: 154! + 1 = 308976961384735088795856467036324046592... : 272 digits
22: 166! - 1 = 900369170577843736647426172359331746074... : 298 digits
23: 320! + 1 = 211610334721925248295571704107762986587... : 665 digits
24: 324! - 1 = 228899746017910232114933690529555743502... : 675 digits
25: 340! + 1 = 510086447210371108093019328392729336303... : 715 digits
26: 379! - 1 = 248403074609647070509353232041657729688... : 815 digits
27: 399! + 1 = 160086307116559738155869925798757514626... : 867 digits
28: 427! + 1 = 290634717696073484110291540071593168467... : 940 digits
29: 469! - 1 = 677180966681495109007809820592109263009... : 1051 digits
30: 546! - 1 = 141302009261418325453762621393526286698... : 1260 digits
Program runtime: 10451693200 ns (~10 seconds)
</pre>
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