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A274626 a(n) = Product_{i=0..2} (2^floor((n+i)/3)-1). 2
0, 0, 0, 1, 3, 9, 27, 63, 147, 343, 735, 1575, 3375, 6975, 14415, 29791, 60543, 123039, 250047, 504063, 1016127, 2048383, 4112895, 8258175, 16581375, 33227775, 66585855, 133432831, 267126783, 534776319, 1070599167, 2142244863, 4286583807, 8577357823, 17158905855, 34326194175, 68669157375 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

Tom Karzes, Jul 05 2016: (Start)

This is a three-dimensional analog of the holes-in-sheet-of-paper sequence A274230.

In d dimensions, assuming the axes for folding are selected in a round-robin fashion, the number of times a given dimension is folded is:

    floor((n+i)/d)

where i runs from 0 (for the last dimension to be folded) through d-1 (for the first dimension to be folded).

The corresponding number of internal dividing lines/planes/etc. is (2^floor((n+i)/d)-1).  The number of internal d-way intersections, which corresponds to the number of holes, is:

    Product_{i=0..d-1}(2^floor((n+i)/d)-1)

where d is the number of dimensions and n is the total number of folds.

Note that the first several nonzero entries in these sequences are the powers of 3. Specifically, in d dimensions, the first d entries are 0, followed by the first (d+1) powers of 3.

It's not hard to see why this is so. The first nonzero entry occurs at d folds, and the value is 1.  This is when you've folded once along each dimension.

After that, the next d folds each divide 2 old partitions into 4 new ones, i.e., they change the internal folds from 1 to 3. So for the next d entries you just multiply the previous entry by 3 (or more generally, by 3/1).

After that you multiply by 7/3 for the next d entries, then 15/7, then 31/15, etc. Each time you're just replacing one of the old factors with a new one, where each factor is one less than a power of two.

Here's an alternative formulation that avoids the iterated product.

For a given number of folds, there are only two factors, each raised to some exponent (with the sum of the exponents being the dimension d):

    v1 = 2^(n/d)-1

    v2 = 2^(n/d+1)-1

    p1 = d-mod(n,d)

    p2 = mod(n,d)

    holes = (v1^p1)*(v2^p2)

This flattens to:

    ((2^(n/d)-1)^(d-mod(n,d))) * ((2^(n/d+1)-1)^(mod(n,d)))

(End)

REFERENCES

Tom Karzes, Posting to Math Fun Mailing List, Jul 05 2016.

LINKS

Table of n, a(n) for n=0..36.

FORMULA

Empirical g.f.: x^3*(1+2*x^2) / ((1-x)*(1-2*x)*(1-2*x^3)*(1-4*x^3)). - Colin Barker, Jul 06 2016

MAPLE

f:=(n, d) ->  mul(2^floor((n+i)/d)-1, i = 0 .. d-1);

[seq(f(n, 3), n=0..40)];

PROG

(PARI) a(n) = prod(i=0, 2, 2^floor((n+i)/3)-1) \\ Colin Barker, Jul 06 2016

CROSSREFS

Cf. A274230, A274627.

Sequence in context: A227097 A201202 A260938 * A161712 A280466 A137368

Adjacent sequences:  A274623 A274624 A274625 * A274627 A274628 A274629

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Jul 05 2016

STATUS

approved

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Last modified August 10 19:52 EDT 2020. Contains 336381 sequences. (Running on oeis4.)