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%I #21 Mar 21 2020 16:39:07
%S 1,127,113,13,5,29,71,7,23,5,10386763,397,37907,73,5,37,13,131,7,5,
%T 278240783,53,8223519074965787731,13,5,7,11,2381,2671,5,31,349,7,151,
%U 5,883,13,11,19,5,521,31,79,4861,5,17,7,17,11,5,47,2618101,709,7,5,219059,17,19,31,5,7,173,13,443,5,269534025881,41,7,1229,5,11,3899827,8699,61,5,13,19
%N a(0) = 1; thereafter a(n) is the smallest prime divisor of the number C(6n+1) formeded from the concatenation of 1,2,3,...,6n+1.
%C A075019(6n + k) for k = {2,3,4,5,6} is {2, 3, 2, 3, 2}.
%C No C(6n+1) has been found to be prime below C(344869).
%F a(n) = A075019(6n+1).
%e For a(1), 1234567 = 127*9721, so a(1) = 127.
%e For a(10), C(61) = A007908(61) = 10386763 * 35280457769357 * 33689963756771087787406890988794422071942750389483226687410462898596940470571223420915460371.
%t f[n_] := Block[{p = 5, s = FromDigits[ Flatten[ IntegerDigits[ Range[ 6n + 1]]]]}, While[ Mod[s, p] > 0, p = NextPrime@ p]; p]; f[0] = 1; Array[ f, 77, 0]
%Y Cf. A007908, A075019.
%K base,nonn
%O 0,2
%A _Robert G. Wilson v_, Oct 15 2018