A227949 Primes obtained by concatenating decremented numbers starting at a power of 10.
109, 10099, 10987, 1000999, 100999897, 10000099999, 1000000999999, 1000999998997, 100000009999999, 10000999999989997, 1000000000999999999, 100999897969594939291, 1000999998997996995994993, 100000000000009999999999999, 10000000000000000000000000099999999999999999999999999
Offset: 1
Examples
Since the concatenation of 10 and 9 produces the prime 109, the number 109 is in the sequence. Since the concatenation of 100 and 99 produces the prime 10099, the number 10099 is in the sequence. The concatenation of 100, 99, 98 does not produce a prime, hence 1009998 is not in the sequence.
Links
- Robert Israel, Table of n, a(n) for n = 1..26
Programs
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Maple
N:= 100: # for terms of up to N digits: S:= NULL: for m from 1 to N/2 do for k from 1 by 2 to 10^m do x:= parse(cat(seq(i,i=10^m .. 10^m - k, -1))); if length(x) > N then break fi; if isprime(x) then S:= S, x; fi od od: sort([S]); # Robert Israel, Jan 23 2024