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A260855 Base-5 representation of a(n) is the concatenation of the base-5 representations of 1, 2, ..., n, n-1, ..., 1. 1

%I

%S 0,1,36,961,24336,3034961,1896581836,1185364159961,740852620019336,

%T 463032888020409961,289395555025471581836,180872221891237629784961,

%U 113045138682031465901269336,70653211676269864870442284961,44158257297668670511080159081836

%N Base-5 representation of a(n) is the concatenation of the base-5 representations of 1, 2, ..., n, n-1, ..., 1.

%C Base-5 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for variants in other bases b = 3, ..., 16.

%C The base 5 is not listed in A260343, because a(5) = A260851(5) = 3034961 is not prime and therefore not in A260852. See these sequences for more information.

%H D. Broadhurst, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;af419558.150">Primes from concatenation: results and heuristics</a>, NmbrThry List, August 1, 2015

%e a(0) = 0 is the result of the empty sum corresponding to 0 digits.

%e a(2) = 36 = (5+1)^2 = 5^2 + 2*5 + 1 = 121_4 is the concatenation of (1, 2, 1).

%e a(5) = 3034961 = 1234104321_5 is the concatenation of (1, 2, 3, 4, 10, 4, 3, 2, 1), where the middle "10" is the base-5 representation of 5.

%o (PARI) a(n,b=5)=sum(i=1,#n=concat(vector(n*2-1,k,digits(min(k,n*2-k),b))),n[i]*b^(#n-i))

%K nonn,base

%O 0,3

%A _M. F. Hasler_, Aug 01 2015

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Last modified June 21 19:12 EDT 2021. Contains 345365 sequences. (Running on oeis4.)