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A336379 Numbers k such that gcd(k, prime(k-1) + prime(k+1)) > 1. 4
4, 5, 6, 8, 9, 10, 11, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 28, 30, 32, 33, 34, 35, 36, 38, 39, 40, 42, 44, 46, 48, 50, 51, 52, 54, 56, 57, 58, 60, 62, 64, 66, 68, 70, 72, 74, 75, 76, 78, 80, 82, 84, 86, 87, 88, 90, 92, 94, 96, 98, 99, 100, 102, 104 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

LINKS

Table of n, a(n) for n=2..66.

EXAMPLE

In the following table, p(k) = A000040(k) = prime(k).

  k    p(k)   p(k-1)+p(k+1)   gcd

  2     3          7           1

  3     5         10           1

  4     7         16           4

  5    11         20           5

  6    13         28           2

2 and 3 are in A336378; 4 and 5 are in this sequence; 3 and 5 are in A336380; 7 and 11 are in A336381.

MATHEMATICA

p[n_] := Prime[n];

u = Select[Range[2, 200], GCD[#, p[# - 1] + p[# + 1]] == 1 &]  (* A336378 *)

v = Select[Range[2, 200], GCD[#, p[# - 1] + p[# + 1]] > 1 &]   (* A336379 *)

Prime[u]  (* A336380 *)

Prime[v]  (* A336381 *)

CROSSREFS

Cf. A000040, A336366, A336378, A336380, A336381.

Sequence in context: A335186 A257816 A033597 * A088720 A134848 A061578

Adjacent sequences:  A336376 A336377 A336378 * A336380 A336381 A336382

KEYWORD

nonn

AUTHOR

Clark Kimberling, Oct 06 2020

STATUS

approved

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Last modified June 22 04:06 EDT 2021. Contains 345367 sequences. (Running on oeis4.)