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A108317
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Smallest a(n) such that a(n) n's plus a(n) is prime, or 0 if no such a(n) exists.
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0
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1, 1, 140, 1, 0, 1, 2, 0, 2, 1, 0, 1, 4, 0, 4, 1, 0, 1, 4, 0, 0, 1, 0, 23, 4, 0, 2, 1, 0, 1, 8, 0, 4198, 497, 0, 1, 2, 0, 8, 1, 0, 1, 0, 0, 2, 1, 0, 35, 2, 0, 2, 1, 0, 0, 2, 0, 4, 1, 0, 1, 2, 0, 4, 17, 0, 1, 64, 0, 2, 1, 0, 1, 14, 0, 2, 0, 0, 1
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OFFSET
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1,3
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COMMENTS
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Some of the larger entries may only correspond to probable primes.
a(n)=0 for n = 3m+2 (1<=m) (they are all divisible by 3) or n=11m+10 (1<=m<9) (they are all divisible by 11) and if a(n) is not 0 then n and a(n) are of opposite parity. - Robert G. Wilson v and Rick L. Shepherd, Jul 28 2005
The sequence continues: 0,4490,1,0,13,14,0,0,1,0,349,10,0,86,2539,0,1,4,0,124,1,0,1,4,0,2,1,0,1,2,0,302,1,0,83,2,0,2,5,0,a(120)>5364,2,0,278,5,0,...,. - Robert G. Wilson v, Jul 28 2005
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LINKS
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FORMULA
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EXAMPLE
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a(13)=4: 4 13s plus 4 = 13131313+4 = 13131317, which is prime.
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MATHEMATICA
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f[n_] := If[(n > 4 && Mod[n, 3] == 2) || (n > 20 && Mod[n, 11] == 10), k = 0, If[n == 1, k = 1, Block[{id = IntegerDigits[n]}, k = Mod[n, 2] + 1; While[ !PrimeQ[ FromDigits[ Flatten[ Table[id, {k}]]] + k], k += 2]]]; k]; Table[ f[n], {n, 100}] (* only good for n<109 *) (* Robert G. Wilson v, Jun 30 2005 *)
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PROG
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(PARI) /* for nonzero terms */ a(n) = m=1; pr=n; while(!isprime(pr+m), m++; pr=eval(concat(Str(pr), n))); m \\ Rick L. Shepherd, Jul 26 2005
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CROSSREFS
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KEYWORD
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base,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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