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%I #9 Sep 10 2017 05:37:57
%S 4,54,549,5491,54999,549993,5499997,55000004,549999972,5499999987,
%T 54999999998,549999999996,5500000000005,55000000000005,
%U 550000000000010,5499999999999987,55000000000000029,549999999999999996
%N a(1) = 4; a(n) = average of the largest and the smallest n-digit primes.
%e a(3) = (997 + 101)/2 = 549.
%p a:=n->(nextprime(10^n-1)+prevprime(10^(n+1)))/2: seq(a(n),n=1..20); # _Emeric Deutsch_, Feb 25 2005
%K base,nonn
%O 1,1
%A _Amarnath Murthy_, Aug 15 2002
%E Corrected and extended by _Emeric Deutsch_, Feb 25 2005
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