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A329794
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a(n) is the smallest positive k such that box(k,n) is a positive square, where box(k,n) is Eric Angelini's mapping defined in the Comments.
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3
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2, 1, 2, 3, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 1, 2, 3, 5, 6, 7, 8, 9, 21, 1, 2, 3, 4, 6, 7, 8, 9, 19, 10, 11, 1, 2, 3, 9, 17, 18, 19, 29, 20, 10, 11, 12, 13, 19, 27, 28, 29, 39, 30, 20, 21, 22, 10, 4, 5, 6, 7, 8, 1
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OFFSET
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1,1
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COMMENTS
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Eric Angelini's "box" map box(i,j) is defined as follows (see A330240). Write i, j in base 10 aligned to the right, say
i = bcd...ef
j = .gh...pq
Then the decimal expansion of box(i,j) is |b-0|, |c-g|, |d-h|, ..., |e-p|, |f-q|.
For example, box(12345,909) = 12644.
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LINKS
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FORMULA
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EXAMPLE
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For n = 1 the smallest k producing a square is 2 (as box(1,2) = 1, this 1 being the square of 1);
For n = 2 the smallest k producing a square is 1 (as box(2,1) = 1, this 1 being the square of 1);
For n = 3 the smallest k producing a square is 2 (as box(3,2) = 1, this 1 being the square of 1);
For n = 5 the smallest k producing a square is 3 (as box(5,1) = 4, this 4 being the square of 2);
For n = 16 the smallest k producing a square is 12 (as box(16,12) = 4, this 4 being the square of 2).
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MATHEMATICA
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BOX[a_, b_]:=FromDigits@Abs[Subtract@@PadLeft[IntegerDigits/@{a, b}]]; Table[k=1; While[!IntegerQ[a=Sqrt@BOX[k, n]]||a==0, k++]; k, {n, 100}] (* Giorgos Kalogeropoulos, Aug 20 2021 *)
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PROG
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(PARI) box(x, y) = if (x==0 || y==0, x+y, 10*box(x\10, y\10) + abs((x%10) - (y%10)))
a(n) = for (k=1, oo, my (b=box(n, k)); if (b && issquare(b), return (b))) \\ Rémy Sigrist, Dec 07 2019
(Python)
from sympy.ntheory.primetest import is_square
def positive_square(n): return n > 0 and is_square(n)
def box(i, j):
si = str(i); sj = str(j); m = max(len(si), len(sj))
si, sj = si.zfill(m), sj.zfill(m)
return int("".join([str(abs(int(si[k])-int(sj[k]))) for k in range(m)]))
def a(n):
k = 1
while not positive_square(box(k, n)): k += 1
return k
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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