A091628 Concatenation of n 2's followed by 3.

23, 223, 2223, 22223, 222223, 2222223, 22222223, 222222223, 2222222223, 22222222223, 222222222223, 2222222222223, 22222222222223, 222222222222223, 2222222222222223, 22222222222222223, 222222222222222223, 2222222222222222223, 22222222222222222223, 222222222222222222223

Offset

1,1

Comments

Sequence arising in Farideh Firoozbakht's solution to Prime Puzzle 251; 23 is the only pointer prime (A089823) not containing the digit "1".

Links

Paolo Xausa, Table of n, a(n) for n = 1..999

Tanya Khovanova, Recursive Sequences

Carlos Rivera's Prime Puzzles and Problems Connection, Puzzle 251, Pointer primes

Index entries for linear recurrences with constant coefficients, signature (11,-10).

Formula

a(n) = (10^(n+1) - 1)/9*2 + 1.

a(n) = 10*a(n-1) - 7, with a(1)=23. - Vincenzo Librandi, Nov 16 2010

From Colin Barker, May 06 2012: (Start)

a(n) = 11*a(n-1) - 10*a(n-2).

G.f.: x*(23-30*x)/((1-x)*(1-10*x)). (End)

Mathematica

(10^Range[2, 25] - 1)/9*2 + 1 (* Paolo Xausa, May 19 2026 *)

Prog

(Magma) [ n eq 1 select 23 else 10*Self(n-1)-7: n in [1..17] ];

Crossrefs

Cf. A089823, A091629, A091630, A091631, A091632.

Sequence in context: A038514 A244766 A125386 * A126544 A272796 A285552

Adjacent sequences: A091625 A091626 A091627 * A091629 A091630 A091631

Keyword

base,easy,nonn

Author

Enoch Haga, Jan 24 2004

Extensions

Edited and extended by Ray Chandler, Feb 07 2004

More terms from Paolo Xausa, May 19 2026

Status

approved