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%I #8 Jul 18 2021 04:42:07
%S 1,2,3,3,6,2,13,13,5,6,2,11,14,15,3,7,77,6,20,41,52,6,13,8,41,52,24,
%T 26,31,41,6,31,21,28,43,57,11,18,47,19,8,33,24,30,43,10,17,213,29,15,
%U 45,60,50,26,48,18,18,78,8,59,13,33,145,23,154,79,65,21,18,23,30,19,159,46,45
%N a(n) is the least k such that k*((prime(n)#)^2)-1 is prime, where prime(n)# is the n-th primorial.
%e 1*2^2-1 = 3 prime, so a(1) = 1.
%e 1*((2*3)^2)-1 = 35 is composite, 2*((2*3)^2)-1 = 71 is prime, so a(2) = 2.
%e 1*((2*3*5)^2)-1 = 899 is composite, 2*((2*3*5)^2)-1 = 1799 is composite, 3*((2*3*5)^2)-1 = 2699 is prime, so a(3) = 3.
%t a[n_] := Module[{k = 1, s = Product[Prime[i], {i,1,n}]^2}, While[!PrimeQ[k*s-1], k++]; k]; Array[a, 75] (* _Amiram Eldar_, Jul 18 2021 *)
%Y Cf. A002110.
%K nonn
%O 1,2
%A _Pierre CAMI_, Mar 20 2005
%E Data corrected by _Amiram Eldar_, Jul 18 2021
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