A178984 a(n) is the smallest prime p that makes prime(n) + 1 - p a prime.
3, 3, 5, 3, 5, 3, 5, 7, 3, 7, 5, 3, 5, 7, 7, 3, 7, 5, 3, 7, 5, 7, 19, 5, 3, 5, 3, 5, 19, 5, 7, 3, 11, 3, 7, 7, 5, 7, 7, 3, 11, 3, 5, 3, 13, 13, 5, 3, 5, 7, 3, 11, 7, 7, 7, 3, 7, 5, 3, 11, 31, 5, 3, 5, 19, 7, 11, 3, 5, 7, 19, 7, 7, 5, 7, 19, 5, 13, 11, 3, 11, 3
Offset: 3
Examples
For n = 3, prime(3) = 5, and a(3) = 3 because 5 + 1 - 3 = 3 is prime; For n = 4, prime(4) = 7, and a(4) = 3 because 7 + 1 - 3 = 5 is prime; ... for n = 25, prime(n) = 97, and a(n) = 19 because 97 + 1 - 19 = 79 is prime.
Crossrefs
Differs from A119912 after 23 terms.
Programs
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Mathematica
f[n_] := Block[{i = 2, p = Prime[n + 2]}, While[q = Prime[i]; ! PrimeQ[p + 1 - q], i++]; q]; Array[f, 60] -
PARI
a(n) = my(p=prime(n), q=2); while (!isprime(p+1-q), q = nextprime(q+1)); q; \\ Michel Marcus, Apr 01 2020
Extensions
More terms from Michel Marcus, Apr 01 2020
Comments