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A158971
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a(n) is the smallest number m such that m n's + 1 is prime and zero if there is no such m.
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1
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1, 1, 0, 1, 0, 1, 0, 2, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 3, 0, 1, 0, 0, 0, 3, 0, 1, 0, 1, 0, 3, 0, 0, 0, 1, 0, 3, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1950, 0, 2, 0, 1, 0, 0, 0, 2, 0, 1, 0, 1, 0, 2, 0, 0, 0, 1, 0, 2, 0, 1, 0, 1, 0, 0, 0, 3, 0, 1, 0, 2, 0, 1, 0, 0, 0, 87, 0, 1, 0, 2, 0, 2, 0, 0, 0, 1, 0, 5, 0, 1, 0, 1, 0, 0
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OFFSET
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1,8
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COMMENTS
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I. If n is an odd number greater than 1 then a(n)=0. II. If n is greater
than 4 and mod(m,10)=4 then a(n)=0. III. If n+1 is prime the a(n)=1.
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LINKS
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EXAMPLE
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20+1 & 2020+1 aren't prime but 202020+1 is prime so a(20)=3. If n>4 and
mod(n,10)=4 then there is no number m such that m n's + 1 is prime because
5 divides all such numbers so a(n)=0.
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MATHEMATICA
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f[n_, m_]:=(v={}; Do[v=Join[v, IntegerDigits[n]], {k, m}]; FromDigits[v]);
a[n_]:=(If[n!=1&&n!=4&&(Mod[n, 10]==4||Mod[n, 2]==1), 0, For[m=1, !PrimeQ[f[n, m]
+1], m++ ]; m]); Do[Print[a[n]], {n, 104}]
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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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