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A048611 Find smallest pair (x,y) such that x^2-y^2 = 11...1 (n times) = (10^n-1)/9; sequence gives value of x. 2
1, 6, 20, 56, 156, 340, 2444, 4440, 167000, 55556, 267444, 333400, 132687920, 5555556, 10731400, 40938800, 2682647040, 333334000, 555555555555555556, 3334367856, 11034444280, 35595935980, 5555555555555555555556 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Least solutions for 'Difference between two squares is a repunit of length n'.

REFERENCES

David Wells, "Curious and Interesting Numbers", Revised Ed. 1997, Penguin Books, p. 119. ISBN 0-14-026149-4.

LINKS

Table of n, a(n) for n=0..22.

H. Havermann, Repunit Square Differences (gives many more terms)

EXAMPLE

For n=2, 6^2-5^2=11.

MATHEMATICA

s = Flatten[Table[r = (10^i - 1)/9; d = Divisors[r]; p = d[[Length[d]/2]]; Solve[{x - y == p, x + y == r/p}, {y, x}], {i, 2, 56}]]; Prepend[Cases[s, Rule[x, n_] -> n], 1]

CROSSREFS

Cf. A048612, A000042, A002275.

Sequence in context: A260777 A014480 A048778 * A200528 A127982 A109164

Adjacent sequences:  A048608 A048609 A048610 * A048612 A048613 A048614

KEYWORD

nonn,nice

AUTHOR

Felice Russo

EXTENSIONS

Corrected and extended by Patrick De Geest, Jun 15 1999. More terms from Hans Havermann, Jul 02 2000

STATUS

approved

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Last modified June 26 01:07 EDT 2017. Contains 288748 sequences.