A281253 Alternately concatenate the decimal digits from front to back 1...n such that n is always to the right.

1, 12, 213, 3124, 42135, 531246, 6421357, 75312468, 864213579, 97531246810, 1086421357911, 119753124681012, 12108642135791113, 1311975312468101214, 141210864213579111315, 15131197531246810121416, 1614121086421357911131517

Offset: 1

Author

Robert G. Wilson v, Jan 18 2017

Keywords

• nonn
• base
• easy

Comments

a(n) is prime for n: 121, 1399 and no others < 3000.

Examples

			a(13) = 12108642135791113.
		

Links

• Indranil Ghosh, Table of n, a(n) for n = 1..340

Crossrefs

Cf. A007908, A138957, A000422, A138793, A281254.

Programs

Maple

b:= proc(n) b(n):= `if`(n=1, 1, parse(cat(n, a(n-1)))) end:
a:= proc(n) a(n):= `if`(n=1, 1, parse(cat(b(n-1), n))) end:
seq(a(n), n=1..20);  # Alois P. Heinz, Jan 18 2017

Mathematica

f[n_] := Fold[ If[ Mod[n +#2, 2] == 0, #1, #2]*10^IntegerLength@ If[ Mod[n +#2, 2] == 0, #2, #1] +If[ Mod[n +#2, 2] == 0, #2, #1] &, 0, Range@ n]; Array[f, 17]

Python

def a(n):
    if n==1:
        return ["1"]
    return a(n-1)[::-1]+[str(n)]
def A281253(n):
    return "".join(a(n)) # Indranil Ghosh, Jan 27 2017