A276200 Largest prime < the concatenation of the numbers from 1 to n (A007908).

11, 113, 1231, 12343, 123449, 1234547, 12345653, 123456761, 12345678899, 1234567891003, 123456789101099, 12345678910111207, 1234567891011121309, 123456789101112131383, 12345678910111213141337, 1234567891011121314151561, 123456789101112131415161717, 12345678910111213141516171723

Offset: 2

Author

Ilya Gutkovskiy, Aug 24 2016

Keywords

• nonn
• base

Examples

			a(5) = 12343, because this is the largest prime less than 12345.
		

Links

• Robert Israel, Table of n, a(n) for n = 2..368
• Eric Weisstein's World of Mathematics, Previous Prime

Crossrefs

Cf. A000040, A000720, A007908, A074365, A151799.

Programs

Maple

tcat:= (a,b) -> a*10^(1+ilog10(b))+b:
t:= 1: R:= NULL:
for i from 2 to 20 do
  t:= tcat(t,i);
  R:= R,prevprime(t);
od:
R; # Robert Israel, Oct 29 2024

Mathematica

Table[NextPrime[FromDigits[Flatten[IntegerDigits[Range[n]]]], -1], {n, 2, 19}]

Formula

a(n) = A151799(A007908(n)).

a(n) = A000040(A000720(A007908(n)-1)).