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

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..26

Formula

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

Example

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.

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

Crossrefs

Cf. A173189, A096594, A172998.

Cf. A227950, A227951.

Sequence in context: A278278 A280626 A357231 * A144930 A190827 A125044

Adjacent sequences: A227946 A227947 A227948 * A227950 A227951 A227952

Keyword

nonn,base

Author

James G. Merickel, Aug 01 2013

Status

approved