A269503 Largest prime factor of A138148(n).

101, 13, 137, 9091, 9901, 909091, 5882353, 52579, 333667, 9091, 99990001, 1058313049, 265371653, 909091, 2906161, 21993833369, 999999000001, 909090909090909091, 1111111111111111111, 909091, 1056689261, 549797184491917, 11111111111111111111111

Offset: 1

Author

Altug Alkan, May 11 2016

Keywords

• nonn

Comments

Largest prime factor of (10^(n+1)+1)*(10^n-1)/9.

Examples

			a(4) = 9091 because largest prime factor of 111101111 is 9091.
		

Links

• Max Alekseyev, Table of n, a(n) for n = 1..344

Crossrefs

Cf. A003020, A003021, A006530, A138148.

Programs

Maple

seq(max(max(numtheory:-factorset((10^n-1)/9)),
max(numtheory:-factorset(10^(n+1)+1))), n=1..30); # Robert Israel, May 11 2016

PARI

a006530(n) = if(n>1, vecmax(factor(n)[, 1]), 1);
for(n=1, 25, print1(a006530((10^(2*n+1)-1)/9-10^n), ", "));

Formula

a(n) = max(A003020(n),A003021(n+1)) for n >= 2. - Robert Israel, May 11 2016