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 A061309 a(n) is the smallest number required to make the concatenation of a(n) and 2n+1 a prime, or -1 if no such number exists. 2
 1, 0, 0, 0, 1, 0, 0, -1, 0, 0, 4, 0, -1, 1, 0, 0, 2, -1, 0, 1, 0, 0, -1, 0, 1, 1, 0, -1, 1, 0, 0, 1, -1, 0, 2, 0, 0, -1, 2, 0, 1, 0, -1, 4, 0, 1, 1, -1, 0, 1, 0, 0, -1, 0, 0, 2, 0, -1, 1, 3, 3, 1, -1, 0, 1, 0, 4, -1, 0, 0, 2, 2, -1, 5, 0, 0, 1, -1, 0, 4, 2, 0, -1, 0, 3, 1, 0, -1, 4, 0, 0, 13 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,11 COMMENTS a(0) = 0 for a prime. LINKS EXAMPLE a(9) = 0 as 19 itself is a prime; a(10) = 4 as 421 is a prime but 121, 221, 321 are not. CROSSREFS Sequence in context: A166317 A068346 A006838 * A263655 A059064 A321316 Adjacent sequences:  A061306 A061307 A061308 * A061310 A061311 A061312 KEYWORD sign,base AUTHOR Amarnath Murthy, Apr 26 2001 EXTENSIONS More terms from Frank Ellermann, Jun 15 2001 STATUS approved

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Last modified October 15 04:33 EDT 2019. Contains 328026 sequences. (Running on oeis4.)