A100897 a(n) is the decimal expansion of 7nn7.

7007, 7117, 7227, 7337, 7447, 7557, 7667, 7777, 7887, 7997, 710107, 711117, 712127, 713137, 714147, 715157, 716167, 717177, 718187, 719197, 720207, 721217, 722227, 723237, 724247, 725257, 726267, 727277, 728287, 729297, 730307

Offset: 0

Author

Parthasarathy Nambi, Jan 10 2005

Keywords

• nonn
• base
• easy

Examples

			If n=6, 7nn7 is 7667.
If n=15, 7nn7 is 715157 (a prime)
If n=18, 7nn7 is 718187 (a prime)
If n=19, 7nn7 is 719197 (a prime)
		

Links

• Daniel Starodubtsev, Table of n, a(n) for n = 0..10000

Crossrefs

Cf. A100846.

Programs

Maple

a:= n-> parse(cat(7,n,n,7)):
seq(a(n), n=0..35);  # Alois P. Heinz, Jan 24 2020

Mathematica

Table[idn=IntegerDigits[n];FromDigits[Join[{7},idn,idn,{7}]],{n,0,30}] (* Harvey P. Dale, May 14 2013 *)

Formula

a(n) = 7nn7.