The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A309120 a(n) is the least k > 1 such that n*k is adjacent to a prime. 2
 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 4, 2, 2, 3, 2, 2, 2, 3, 2, 2, 2, 2, 2, 3, 2, 2, 2, 2, 4, 2, 2, 3, 6, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 6, 3, 6, 5, 2, 2, 2, 2, 4, 2, 2, 2, 4, 5, 4, 2, 2, 2, 4, 2, 2, 3, 2, 2, 2, 2, 6, 2, 2, 3, 2, 2, 2, 3, 4, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If n is odd then a(n) is even. a(n) exists by Dirichlet's theorem on primes in arithmetic progressions. LINKS Robert Israel, Table of n, a(n) for n = 1..10000 FORMULA a(A104278(n)) > 2 and a(A147820(n)) = 2. - Ivan N. Ianakiev, Jul 18 2019 EXAMPLE a(13)=4 because 4*13+1=53 is prime but none of 2*13-1,2*13+1,3*13-1,3*13+1 are primes. MAPLE f:= proc(m) local k;   for k from 2 by 1+(m mod 2) do     if isprime(k*m-1) or isprime(k*m+1) then return k fi   od end proc: map(f, [\$1..100]); MATHEMATICA a[n_]:=Module[{k=2}, While[Not[PrimeQ[k*n-1]||PrimeQ[k*n+1]], k++]; k]; a/@Range[94] (* Ivan N. Ianakiev, Jul 18 2019 *) PROG (PARI) a(n) = my(k=2); while (!isprime(n*k+1) && !isprime(n*k-1), k++); k; \\ Michel Marcus, Jul 19 2019 CROSSREFS Cf. A307833, A104278, A147820. Sequence in context: A146167 A346622 A103380 * A098708 A067394 A337301 Adjacent sequences:  A309117 A309118 A309119 * A309121 A309122 A309123 KEYWORD nonn AUTHOR Robert Israel, Jul 17 2019 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified January 20 03:25 EST 2022. Contains 350467 sequences. (Running on oeis4.)