Factorial primes: Difference between revisions
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31: 546! - 1 = 14130200926141832545...99999999999999999999 (1260 digits)
</pre>
=={{header|Delphi}}==
{{works with|Delphi|6.0}}
{{libheader|SysUtils,StdCtrls}}
<syntaxhighlight lang="Delphi">
function IsPrime(N: int64): boolean;
{Optimised prime test - about 40% faster than the naive approach}
var I,Stop: integer;
begin
if (N = 2) or (N=3) then Result:=true
else if (n <= 1) or ((n mod 2) = 0) or ((n mod 3) = 0) then Result:= false
else
begin
I:=5;
Stop:=Trunc(sqrt(N*1.0));
Result:=False;
while I<=Stop do
begin
if ((N mod I) = 0) or ((N mod (i + 2)) = 0) then exit;
Inc(I,6);
end;
Result:=True;
end;
end;
function Factorial(N: Word): int64;
var I: integer;
begin
Result:= 1;
for I := 2 to N do Result:=Result * I;
end;
procedure ShowFactorialPrimes(Memo: TMemo);
{Show factorials where F+1 or F-1 are prime}
var I,Cnt: integer;
var F: int64;
procedure DisplayItem(Minus: boolean);
var S: string;
var Sign: char;
var F1: int64;
begin
Inc(Cnt);
if Minus then F1:=F-1 else F1:=F+1;
if Minus then Sign:='-' else Sign:='+';
S:=Format('%2d: %3d! %s 1 = %d',[Cnt,I,Sign,F1]);
Memo.Lines.Add(S);
end;
begin
Cnt:=0;
for I:=1 to High(Integer) do
begin
F:=Factorial(I);
if IsPrime(F+1) then DisplayItem(False);
if IsPrime(F-1) then DisplayItem(True);
if Cnt>=10 then break;
end;
end;
</syntaxhighlight>
{{out}}
<pre>
1: 1! + 1 = 2
2: 2! + 1 = 3
3: 3! + 1 = 7
4: 3! - 1 = 5
5: 4! - 1 = 23
6: 6! - 1 = 719
7: 7! - 1 = 5039
8: 11! + 1 = 39916801
9: 12! - 1 = 479001599
10: 14! - 1 = 87178291199
</pre>
=={{header|F_Sharp|F#}}==
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