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 A173515 Consider positive integer solutions to x^3+ y^3 = z^3 - n or 'Fermat near misses' of 1, 2, 3 ... Arrange known solutions by increasing values of n. Sequence gives value of lowest z for a given n. 2
 9, 7, 2, 812918, 18, 217, 4, 3, 9730705, 332, 14, 135, 3, 19, 156, 16, 15584139827, 3, 139643, 6, 1541, 4, 2220422932, 5, 14, 4, 445, 12205, 9, 8, 16234, 815, 31, 4 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The submitted values are for z when 0 < n < 51. There is no solution for any n congruent to 4 or 5 mod 9. This eliminates 4,5,13,14,22,23,31,32,40,41,49 and 50 in the 0 to 50 range. Per the Elsenhans and Jahnel link there are no solutions found for 3, 33, 39 and 42 in the 0 to 50 range, with a search bound of 10^14. If sequences could contain 'nil' for no solution, and '?' for cases where a solution is not known, but might exist, then a more concise definition is possible: Least positive integer such that a(n)^3 - n is the sum of two positive cubes. The sequence would then start with: 9, 7, ?, nil, nil, 2 LINKS Eric S. Rowland, Known Families of Integer Solutions of x^3 + y^3 + z^3 = n Kenji Koyama, Yukio Tsuruoka, and Hiroshi Sekigawa, On Searching For Solutions of the Diophantine Equation x^3 + y^3 + z^3 = n, Math. Comp. 66 (1997), 841-851. D.R. Heath-Brown, W.M. Lioen and H.J.J. te Riele, on Solving the Diophantine Equation x^3 + y^3 + z^3 = k on a Vector Computer Andreas-Stephan Elsenhans and Joerg Jahnel, List of solutions of x^3 + y^3 + z^3 = n for n < 1000 neither a cube nor twice a cube FORMULA Author conjectures that explicit formula or recurrence does not exist. EXAMPLE 6^3 + 8^3 = 9^3 - 1 There are no solutions when n = 1 for z < 9, thus the first term is 9. 5^3 + 6^3 = 7^3 - 2 There are no solution for z < 7, thus the second term is 7. It is unknown if there is a solution when n = 3. It is known there are no solutions when n = 4 and 5. 1^3 + 1^3 = 2^3 - 6, this the third term is 2. PROG (Ruby) # x^3 + y^3 = z^3 - n # Solve for all z less than z_limit, and # n less than n_limit. # When n = 7, z = 812918 and faster code and language are needed. # However, by optimizing this code slightly and running for 2 days # the author was able to search all z < 164000 and n < 100 # n_limit = 7 # Configure as desired z_limit = 20 # Configure as desired h = {} (2..z_limit).each{ |z| . (1..(z-1)).each{ |y| . (1..(y)).each{ |x| . n = z*z*z - x*x*x - y*y*y . if n > 0 && n < n_limit && h[n].nil? . puts "Found z = #{z} when #{x}^^3 + #{y}^^3 = #{z}^^3 - #{n}" . h[n] = z . end } } } print "\nPartial sequence generated when n < #{n_limit} and z is searched to #{z_limit} is:\n" h.sort.each{|k, v| print "#{v}, " } print "\b\b \n" CROSSREFS Cf. A050788, A159935 Sequence in context: A155792 A247226 A197834 * A091558 A244659 A307229 Adjacent sequences:  A173512 A173513 A173514 * A173516 A173517 A173518 KEYWORD nonn AUTHOR Andy Martin, Feb 20 2010 STATUS approved

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Last modified December 7 09:33 EST 2019. Contains 329843 sequences. (Running on oeis4.)