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 A072182 A Wallis pair (x,y) satisfies sigma(x^2) = sigma(y^2); sequence gives x's for Wallis pairs with x < y (ordered by values of x, then y). 5
 4, 12, 28, 36, 44, 52, 68, 76, 84, 92, 108, 116, 124, 132, 148, 156, 164, 172, 188, 196, 204, 212, 228, 236, 244, 252, 268, 276, 284, 292, 308, 316, 324, 326, 332, 348, 356, 364, 372, 388, 396, 404, 406, 412, 428, 436, 444, 452, 468, 476, 484, 492, 508, 516 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 4*A045572 is included in this sequence. - Benoit Cloitre, Oct 22 2002 D. Johnson remarks that some terms are repeated, e.g., a(139)=a(140)=1284 forms a Wallis pair with A072186(139)=1528 and also with A072186(140)=1605. - M. F. Hasler, Sep 15 2013 REFERENCES I. Kaplansky, The challenges of Fermat, Wallis and Ozanam (and several related challenges): II. Fermat's second challenge, Preprint, 2002. LINKS Donovan Johnson, Table of n, a(n) for n = 1..10000 EXAMPLE The first few pairs are all multiples of the first pair (4,5): (4, 5), (12, 15), (28, 35), (36, 45), (44, 55), (52, 65), ... MATHEMATICA w = {}; m = 550; Do[q = DivisorSigma[1, x^2]; sq = Sqrt[q] // Floor; Do[If[q == DivisorSigma[1, y^2], AppendTo[w, {x, y}]], {y, x+1, sq}], {x, 1, m}]; w[[All, 1]] (* Jean-François Alcover, Oct 01 2019 *) PROG (PARI) {w=[]; m=550; for(x=1, m, q=sigma(x^2); sq=sqrtint(q); for(y=x+1, sq, if(q==sigma(y^2), w=concat(w, [[x, y]])))); for(j=1, matsize(w)[2], print1(w[j][1], ", "))} (Haskell) a072182 n = a072182_list !! (n-1) (a072182_list, a072186_list) = unzip wallisPairs   wallisPairs = [(x, y) | (y, sy) <- tail ws,                           (x, sx) <- takeWhile ((< y) . fst) ws, sx == sy]                 where ws = zip [1..] \$ map a000203 \$ tail a000290_list -- Reinhard Zumkeller, Sep 17 2013 CROSSREFS Cf. A072186, A075768, A075769. Cf. A000203, A000290. Sequence in context: A212522 A207408 A064444 * A009906 A194432 A220512 Adjacent sequences:  A072179 A072180 A072181 * A072183 A072184 A072185 KEYWORD nonn,easy AUTHOR N. J. A. Sloane, Oct 19 2002 EXTENSIONS Extended by Klaus Brockhaus and Benoit Cloitre, Oct 22 2002 STATUS approved

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Last modified December 15 20:00 EST 2019. Contains 330000 sequences. (Running on oeis4.)