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%I #35 May 01 2023 10:03:09
%S 1,221,333221,4444333221,555554444333221,666666555554444333221,
%T 7777777666666555554444333221,888888887777777666666555554444333221,
%U 999999999888888887777777666666555554444333221,10101010101010101010999999999888888887777777666666555554444333221
%N a(n) is the concatenation of n n times, n-1 n-1 times, ..., 22, 1.
%C a(n) is composite for all 2 <= n <= 1000. - _David Cleaver_, Apr 14 2023
%H Alois P. Heinz, <a href="/A300517/b300517.txt">Table of n, a(n) for n = 1..31</a>
%e a(4) = 4444333221 because 4 concatenated four times then concatenated with 3 three times and 2 twice and 1 once gives 4444333221.
%p a:= n-> parse(cat(seq((n-i)$(n-i), i=0..n-1))):
%p seq(a(n), n=1..12); # _Alois P. Heinz_, Mar 07 2018
%o (Ruby)
%o def A300517(n)
%o a = '1'
%o [1] + (2..n).map{|i| a = (i.to_s * i + a.to_s).to_i}
%o end
%o p A300517(20)
%o (PARI)
%o a(n) = {my(a=0, b=0, d=1, i);
%o for(i=1, n, b = logint(i, 10)+1;
%o a += d*i*(10^(i*b)-1)/(10^b-1);
%o d *= 10^(i*b); ); return(a); } \\ _David Cleaver_, Apr 14 2023
%Y Cf. A000461, A098129, A361751 (number of decimal digits).
%K nonn,base
%O 1,2
%A _Seiichi Manyama_, Mar 07 2018
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