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%I #13 Jun 29 2019 11:23:44
%S 0,1,3721,13402921,48250954921,173703464074921,625332472251274921,
%T 2251196900199483274921,8104308840723833403274921,
%U 29175511826606141868603274921,105031842575782131223980603274921,378114633272815673636150700603274921
%N Base-60 representation of a(n) is the concatenation of the base-60 representations of 1, 2, ..., n, n-1, ..., 1.
%C See A260343 for the bases b such that A260851(b) = A_b(b) = b*c + (c - b)*(1 + b*c), is prime, where A_b is the base-b sequence, as here with b=60, and c = R(b,b) = (b^b-1)/(b-1) is the base-b repunit of length b.
%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
%F For n < b = 60, we have a(n) = A_b(n) = R(b,n)^2, where R(b,n) = (b^n-1)/(b-1) are the base-b repunits.
%e a(0) = 0 is the result of the empty sum corresponding to 0 digits.
%e a(2) = (60+1)^2 = 60^2 + 2*60 + 1 = 121_60, concatenation of (1, 2, 1).
%e a(61) = 123...101110...21_60, which is the concatenation of (1, 2, 3, ..., 10, 11, 10, ..., 2, 1), where the middle "10, 11, 10" are the base-60 representations of 60, 61, 60.
%o (PARI) a(n,b=60)=sum(i=1,#n=concat(vector(n*2-1,k,digits(min(k,n*2-k),b))),n[i]*b^(#n-i))
%Y Base-60 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for variants in other bases.
%K nonn,base
%O 0,3
%A _M. F. Hasler_, Aug 01 2015
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