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A260861 Base-11 representation of a(n) is the concatenation of the base-11 representations of 1, 2, ..., n, n-1, ..., 1.

%I A260861 #11 Jun 29 2019 11:26:50
%S A260861 0,1,144,17689,2143296,259371025,31384248336,3797497946089,
%T A260861 459497294348544,55599173087763361,6727499948806851600,
%U A260861 8954302429379707945271,131099941868210323821706774,1919434248892467772593071038679,28102436838034620750856132266604106
%N A260861 Base-11 representation of a(n) is the concatenation of the base-11 representations of 1, 2, ..., n, n-1, ..., 1.
%C A260861 The first prime in this sequence is a(13) = A260871(9). Since a(11) is not prime, the base 11 is not listed in A260343.
%H A260861 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
%F A260861 For n < b = 11, we have a(n) = R(b,n)^2, where R(b,n) = (b^n-1)/(b-1) are the base-b repunits.
%e A260861 a(0) = 0 is the result of the empty sum corresponding to 0 digits.
%e A260861 a(2) = (11+1)^2 = 11^2 + 2*11 + 1 = 121_11, concatenation of (1, 2, 1).
%e A260861 a(12) = 123456789a101110a987654321_11 is the concatenation of (1, 2, 3, ..., 9, a, 10, 11, 10, a, 9, ..., 1), where "a, 10, 11" are the base-11 representations of 10, 11, 12.
%o A260861 (PARI) a(n,b=11)=sum(i=1,#n=concat(vector(n*2-1,k,digits(min(k,n*2-k),b))),n[i]*b^(#n-i))
%Y A260861 Base-11 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for variants in other bases.
%K A260861 nonn,base
%O A260861 0,3
%A A260861 _M. F. Hasler_, Aug 01 2015