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 A263920 A positive integer n is in this sequence iff arctan(n)^2 can be represented as Sum_{0
 7, 47, 57, 99, 117 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The terms given are certainly in the sequence. Although I lack a rigorous proof that no intermediate terms were omitted, an extensive computer search gave no other candidates in between. It is an open question if the sequence is infinite. LINKS Eric Weisstein's MathWorld, Inverse Tangent. EXAMPLE 7 is in the sequence, because arctan(7)^2 = -5*arctan(1)^2 + (10/3)*arctan(2)^2 + (2/3)*arctan(3)^2. 47 is in the sequence, because arctan(47)^2 = (2939/210)*arctan(2)^2 - (125/21)*arctan(3)^2 - (6/5)*arctan(4)^2 - (12/7)*arctan(5)^2 - (29/7)*arctan(7)^2 + (15/7)*arctan(8)^2 + (2/5)*arctan(13)^2 + (11/7)*arctan(18)^2 - arctan(21)^2 + (7/10)*arctan(38)^2. CROSSREFS Cf. A005528, A002312. Sequence in context: A089725 A086040 A009241 * A124837 A122731 A059452 Adjacent sequences:  A263917 A263918 A263919 * A263921 A263922 A263923 KEYWORD nonn,hard,more AUTHOR Vladimir Reshetnikov, Oct 29 2015 STATUS approved

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Last modified September 20 20:02 EDT 2020. Contains 337265 sequences. (Running on oeis4.)