A113574 a(n) is the least n-digit number whose k-th digit is prime if k is prime, composite if k is composite, and 1 if k=1.

1, 12, 122, 1224, 12242, 122424, 1224242, 12242424, 122424244, 1224242444, 12242424442, 122424244424, 1224242444242, 12242424442424, 122424244424244, 1224242444242444, 12242424442424442, 122424244424244424

Offset

1,2

Links

Alvin Hoover Belt, Table of n, a(n) for n = 1..75

Formula

a(n+1) = a(n)||2 if n+1 is prime, otherwise a(n+1) = a(n)||4 where || is concatenation.

Mathematica

nxt[{n_, a_}]:={n+1, If[PrimeQ[n+1], 10a+2, 10a+4]}; NestList[nxt, {1, 1}, 20][[All, 2]] (* Harvey P. Dale, Jan 16 2023 *)

Prog

(Python)
from sympy import isprime
def a(n): return 1 if n<2 else int(str(a(n - 1)) + "2") if isprime(n) else int(str(a(n - 1)) + "4")
print([a(n) for n in range(1, 51)]) # Indranil Ghosh, Apr 12 2017

Crossrefs

Sequence in context: A134584 A090843 A132927 * A255307 A199592 A285807

Adjacent sequences: A113571 A113572 A113573 * A113575 A113576 A113577

Keyword

base,easy,nonn

Author

Amarnath Murthy, Nov 06 2005

Extensions

Corrected and extended by Alvin Hoover Belt, Mar 24 2008

Name edited by Jon E. Schoenfield, Mar 20 2021

Status

approved