Erdős-primes: Difference between revisions

=={{header|C#|CSharp}}==▼

<lang csharp>using System; using static System.Console;▼

class Program {▼

const int lmt = (int)1e6, first = 2500; static int[] f = new int[10];▼

static void Main(string[] args) {▼

f[0] = 1; for (int a = 0, b = 1; b < f.Length; a = b++)▼

f[b] = f[a] * (b + 1);▼

int pc = 0, nth = 0, lv = 0;▼

for (int i = 2; i < lmt; i++) if (is_erdos_prime(i)) {▼

if (i < first) Write("{0,5:n0}{1}", i, pc++ % 5 == 4 ? "\n" : " ");▼

nth++; lv = i; }▼

Write("\nCount of Erdős primes between 1 and {0:n0}: {1}\n{2} Erdős prime (the last one under {3:n0}): {4:n0}", first, pc, card(nth), lmt, lv); }▼

static string card(int n) {▼

return string.Format("{0:n0}", n) + new string[]{"th", "st", "nd", "rd", "th", "th", "th", "th", "th", "th"}[n % 10]; }▼

static bool is_erdos_prime(int p) {▼

if (!is_pr(p)) return false; int m = 0, t;▼

while ((t = p - f[m++]) > 0) if (is_pr(t)) return false;▼

return true;▼

bool is_pr(int x) {▼

if (x < 4) return x > 1; if ((x & 1) == 0) return false;▼

for (int i = 3; i * i <= x; i += 2) if (x % i == 0) return false;▼

return true; } } }</lang>▼

{{out}}▼

<pre> 2 101 211 367 409▼

419 461 557 673 709▼

769 937 967 1,009 1,201▼

1,259 1,709 1,831 1,889 2,141▼

2,221 2,309 2,351 2,411 2,437▼

Count of Erdős primes between 1 and 2,500: 25▼

7,875th Erdős prime (the last one under 1,000,000): 999,721</pre>▼