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%I #25 Feb 19 2023 16:24:21
%S 0,1,16,439,35350,2864599,232046890,18795930559,1522471570630,
%T 369960528437035,269701223137448146,196612191672080116867,
%U 143330287729139571972130,104487779754548024866115515,76171591441065652665051372946,55529090160536864641400481743827
%N Base-3 representation of a(n) is the concatenation of the base-3 representations of 1, 2, ..., n, n-1, ..., 1.
%C The base 3 is listed in A260343, which means that a(3) = A260851(3) = 439 = 121021_3 is prime and therefore in A260852. See these sequences for more information.
%H D. Broadhurst, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;af419558.1508">Primes from concatenation: results and heuristics</a>, NmbrThry List, August 1, 2015
%H <a href="/index/Mo#MWP">Index entries for sequences related to Most Wanted Primes video</a>
%e a(0) = 0 is the result of the empty sum corresponding to 0 digits.
%e a(2) = 16 = 121_3 is the concatenation of (1, 2, 1).
%e a(3) = 439 = 121021_3 is the concatenation of (1, 2, 10, 2, 1), where the middle "10" is the base-3 representation of 3.
%t Join[{0},Table[FromDigits[Join[Flatten[IntegerDigits[Range[n], 3]], Flatten[ Reverse[ Most[ IntegerDigits[Range[n],3]]]]],3],{n,20}]] (* _Harvey P. Dale_, Mar 11 2019 *)
%o (PARI) a(n,b=3)=sum(i=1,#n=concat(vector(n*2-1,k,digits(min(k,n*2-k),b))),n[i]*b^(#n-i))
%Y Base-3 variant of A173426 (base 10) and A173427 (base 2). See A260854 - A260866 for variants in other bases b = 4, ..., 16.
%K nonn,base
%O 0,3
%A _M. F. Hasler_, Aug 01 2015
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