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A241424
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Largest number k > 0 such that n - k! is prime, or 0 if no such k exists.
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2
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0, 0, 1, 2, 2, 1, 2, 3, 3, 0, 3, 1, 3, 1, 2, 0, 3, 1, 3, 1, 2, 0, 3, 1, 3, 4, 4, 0, 4, 1, 4, 1, 2, 0, 4, 0, 4, 1, 2, 0, 4, 1, 4, 1, 2, 0, 4, 1, 3, 0, 0, 0, 4, 1, 4, 0, 0, 0, 3, 1, 4, 1, 2, 0, 4, 0, 4, 1, 2, 0, 4, 1, 3, 1, 2, 0, 4, 0, 3, 1, 2, 0, 4, 1, 4, 0, 0, 0, 3, 1, 4, 0, 0, 0, 4
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OFFSET
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1,4
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COMMENTS
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If k > n, n - k! is negative and therefore, not prime.
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LINKS
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MAPLE
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a:= proc(n) local k, r;
r:= 0;
for k from 1 do
if k! >= n then return r
elif isprime(n-k!) then r:= k
fi
od
end proc:
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MATHEMATICA
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a[n_] := Module[{k, r = 0}, For[k = 1, True, k++, If[k! >= n, Return[r], If[PrimeQ[n - k!], r = k]]]];
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PROG
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(PARI)
a(n)=forstep(k=n, 1, -1, if(ispseudoprime(n-k!), return(k)))
n=1; while(n<150, print1(a(n), ", "); n++)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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