OFFSET
1,1
COMMENTS
For a 2-digit integer ab, the "center" is the thin space between a and b; the digit immediately to the right of the center is thus b;
For a 3-digit integer abc, the "center" is the digit b; the digit immediately to the right of the center is thus c;
For a 4-digit integer abcd, the "center" is the thin space between b and c; the digit immediately to the left of the center is thus c;
For a 5-digit integer abcde, the "center" is the digit c; the digit immediately to the left of the center is thus d; etc.
EXAMPLE
The first twelve terms of the sequence are:
11, 21, 12, 31, 41, 22, 13, 51, 14, 61, 32, 42.
We put parentheses around the digit right of center:
1(1), 2(1), 1(2), 3(1), 4(1), 2(2), 1(3), 5(1), 1(4), 6(1), 3(2), 4(2).
The twelve digits in parentheses are:
1, 1, 2, 1, 1, 2, 3, 1, 4, 1, 2, 2.
The above twelve digits are the same as the first twelve digits of the sequence:
11, 21, 12, 31, 41, 22.
MATHEMATICA
a[1]=11; a[n_]:=a[n]=(k=10; While[MemberQ[ar=Array[a, n-1], k]||IntegerDigits[k][[Ceiling[IntegerLength@k/2]+1]]!=Flatten[Join[Flatten[IntegerDigits/@ar], IntegerDigits@k]][[n]], k++]; k); Array[a, 70] (* Giorgos Kalogeropoulos, Sep 21 2023 *)
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Eric Angelini, Sep 16 2023
EXTENSIONS
More terms from Giorgos Kalogeropoulos, Sep 21 2023
STATUS
approved