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A227949 Primes obtained by concatenating decremented numbers starting at a power of 10.

%I A227949 #35 Jan 24 2024 08:30:25
%S A227949 109,10099,10987,1000999,100999897,10000099999,1000000999999,
%T A227949 1000999998997,100000009999999,10000999999989997,1000000000999999999,
%U A227949 100999897969594939291,1000999998997996995994993,100000000000009999999999999,10000000000000000000000000099999999999999999999999999
%N A227949 Primes obtained by concatenating decremented numbers starting at a power of 10.
%H A227949 Robert Israel, <a href="/A227949/b227949.txt">Table of n, a(n) for n = 1..26</a>
%F A227949 A227949(n) begins with 10^[(A227950(n)-1)/A227951(n)].
%e A227949 Since the concatenation of 10 and 9 produces the prime 109, the number 109 is in the sequence.
%e A227949 Since the concatenation of 100 and 99 produces the prime 10099, the number 10099 is in the sequence.
%e A227949 The concatenation of 100, 99, 98 does not produce a prime, hence 1009998 is not in the sequence.
%p A227949 N:= 100: # for terms of up to N digits:
%p A227949 S:= NULL:
%p A227949 for m from 1 to N/2 do
%p A227949   for k from 1 by 2 to 10^m do
%p A227949     x:= parse(cat(seq(i,i=10^m .. 10^m - k, -1)));
%p A227949     if length(x) > N then break fi;
%p A227949     if isprime(x) then S:= S, x;  fi
%p A227949 od od:
%p A227949 sort([S]); # _Robert Israel_, Jan 23 2024
%Y A227949 Cf. A173189, A096594, A172998.
%Y A227949 Cf. A227950, A227951.
%K A227949 nonn,base
%O A227949 1,1
%A A227949 _James G. Merickel_, Aug 01 2013