A227949 - OEIS

%I #35 Jan 24 2024 08:30:25

%S 109,10099,10987,1000999,100999897,10000099999,1000000999999,

%T 1000999998997,100000009999999,10000999999989997,1000000000999999999,

%U 100999897969594939291,1000999998997996995994993,100000000000009999999999999,10000000000000000000000000099999999999999999999999999

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

%H Robert Israel, <a href="/A227949/b227949.txt">Table of n, a(n) for n = 1..26</a>

%F A227949(n) begins with 10^[(A227950(n)-1)/A227951(n)].

%e Since the concatenation of 10 and 9 produces the prime 109, the number 109 is in the sequence.

%e Since the concatenation of 100 and 99 produces the prime 10099, the number 10099 is in the sequence.

%e The concatenation of 100, 99, 98 does not produce a prime, hence 1009998 is not in the sequence.

%p N:= 100: # for terms of up to N digits:

%p S:= NULL:

%p for m from 1 to N/2 do

%p   for k from 1 by 2 to 10^m do

%p     x:= parse(cat(seq(i,i=10^m .. 10^m - k, -1)));

%p     if length(x) > N then break fi;

%p     if isprime(x) then S:= S, x;  fi

%p od od:

%p sort([S]); # _Robert Israel_, Jan 23 2024

%Y Cf. A173189, A096594, A172998.

%Y Cf. A227950, A227951.

%K nonn,base

%O 1,1

%A _James G. Merickel_, Aug 01 2013