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A100986
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Smallest k such that concatenation of r*k and 1 is a prime for all r = 1 to n but not prime for r = n+1, or smallest k such that 10*r*k+1 is a prime for all r = 1 to n but not prime for r = n+1.
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9
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OFFSET
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1,2
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COMMENTS
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LINKS
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EXAMPLE
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a(4)=33 because 331, 661, 991 and 1321 (1321=10*4*33+1) are all prime, but 1651 (1651=10*5*33+1) is not prime. - Robert Price, Apr 02 2019
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MATHEMATICA
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Table[k = 1;
While[! AllTrue[Table[10*r*k + 1, {r, 1, n}], PrimeQ] ||
PrimeQ[10*(n + 1)*k + 1], k++]; k, {n, 1, 9}] (* Robert Price, Apr 02 2019 *)
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PROG
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(PARI) isok(k, n) = {for (r=1, n, if (! isprime(10*r*k+1), return (0)); ); !isprime(10*(n+1)*k+1); }
a(n) = {my(k=1); while(! isok(k, n), k++); k; } \\ Michel Marcus, Apr 03 2019
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CROSSREFS
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KEYWORD
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more,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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