A241425 Largest number k > 0 such that n + k! and n - k! are both prime, or 0 if no such k exists.
0, 0, 0, 1, 2, 1, 0, 0, 2, 0, 3, 1, 3, 0, 2, 0, 3, 1, 0, 0, 2, 0, 3, 0, 3, 0, 0, 0, 4, 1, 0, 0, 0, 0, 4, 0, 4, 0, 2, 0, 0, 1, 4, 0, 2, 0, 4, 0, 0, 0, 0, 0, 3, 0, 4, 0, 0, 0, 0, 1, 0, 0, 0, 0, 4, 0, 3, 0, 2, 0, 0, 1, 3, 0, 0, 0, 4, 0, 0, 0, 2, 0, 4, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 3
Offset: 1
Keywords
Links
- Robert Israel, Table of n, a(n) for n = 1..10000
Programs
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Maple
a:= proc(n) local k; for k from min(numtheory:-factorset(n))-1 to 1 by -1 do if n > k! and isprime(n+k!) and isprime(n-k!) then return(k) fi od: 0 end proc: a(1):= 0: seq(a(n),n=1..100); # Robert Israel, Aug 10 2014
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Mathematica
a[n_] := Module[{k}, For[k = FactorInteger[n][[1, 1]], k >= 1, k--, If[n > k! && PrimeQ[n + k!] && PrimeQ[n - k!], Return[k]]]; 0]; a[1] = 0; Array[a, 100] (* Jean-François Alcover, Jul 27 2020, after Maple *) -
PARI
a(n)=forstep(k=n,1,-1,if(ispseudoprime(n+k!)&&ispseudoprime(n-k!),return(k))) n=1;while(n<150,print1(a(n),", ");n++)
Comments