A071407 - OEIS

%I #18 Oct 15 2013 22:31:23

%S 2,2,6,6,18,24,6,12,6,12,42,54,30,24,6,120,18,258,24,18,84,132,54,48,

%T 114,42,6,6,48,24,144,30,6,12,12,78,24,36,30,54,132,18,90,36,66,18,42,

%U 30,120,30,36,42,18,18,54,84,60,12,210,12,6,60,150,102,6,210,30,24,6

%N Least k such that k*prime(n) + 1 and k*prime(n) - 1 are twin primes.

%C Note that 6 divides a(n) for n > 2. - _T. D. Noe_, Jan 07 2013

%H T. D. Noe, <a href="/A071407/b071407.txt">Table of n, a(n) for n = 1..10000</a>

%e n=4: prime(4)=7, a(4)=6 because 6*prime(4)=42 and {41,43} are primes.

%t Table[fl=1; Do[s=(Prime[j])*k; If[PrimeQ[s-1]&&PrimeQ[s+1]&&Equal[fl, 1], Print[{j, k}]; fl=0], {k, 1, 2*j^2}], {j, 0, 100}]

%o (Haskell)

%o a071407 n = head [k | k <- [2,4..], let x = k * a000040 n,

%o                       a010051' (x - 1) == 1, a010051' (x + 1) == 1]

%o -- _Reinhard Zumkeller_, Feb 14 2013

%Y Cf. A071256, A071404-A071406, A060256, A060210.

%Y Cf. A071558 (k at every integer).

%Y Cf. A220141, A220142 (record values).

%K nonn

%O 1,1

%A _Labos Elemer_, May 24 2002