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%I #8 Oct 20 2019 17:26:55
%S 21,117,241,283,389,517,547,627,1219,1223,1247,1285,1287,1323,1467,
%T 1505,1591,1599,1689,1713,1817,1961,2203,2309,2377,2607,2837,2847,
%U 2899,2911,3227,3261,3499,3823,4007,4069,4183,4347,4403,4473,4507,4535,4637
%N Concatenation of n in base 10 down up to base 2 is prime, all numbers are interpreted as decimals.
%H Harvey P. Dale, <a href="/A054257/b054257.txt">Table of n, a(n) for n = 0..1000</a>
%e a(1)=21 -> 21{10}=23{9}=25{8}=30{7}=33{6}=41{5}=111{4}=210{3}=10101{2} -> 21232530334111121010101 is a prime.
%t Select[Range[5000],PrimeQ[FromDigits[Flatten[Table[IntegerDigits[#,n],{n,10,2,-1}]]]]&] (* _Harvey P. Dale_, May 25 2012 *)
%Y Cf. A054256, A054258.
%K nonn,base
%O 0,1
%A _Patrick De Geest_, Feb 15 2000