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 A281816 Least k such that phi(k) is the sum of two totient numbers (A002202) in exactly n ways, or 0 if no such k exists. 1
 1, 3, 11, 13, 23, 29, 37, 41, 81, 53, 67, 61, 73, 97, 103, 89, 109, 143, 139, 113, 137, 157, 149 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS For the first 10000 terms of A281687, only A281687(93) = 23 and 2 * 93 = 186 is not a totient number. With this observation if we consider the scatterplot of A281687, a(23) is probably equal to 0, but this is still unproved at this moment. So this sequence has keyword "more". a(24) - a(71) are 173, 181, 193, 235, 247, 301, 271, 229, 253, 289, 233, 519, 269, 281, 293, 337, 317, 439, 349, 397, 373, 353, 409, 575, 535, 433, 401, 571, 389, 449, 551, 461, 879, 623, 577, 743, 521, 509, 557, 685, 689, 569, 661, 593, 767, 709, 653, 641. LINKS EXAMPLE a(3) = 13 because phi(13) = 12 = 2 + 10 = 4 + 8 = 6 + 6; 2, 4, 6, 8, 10 are in A002202 and 13 is the least number with this property. PROG (PARI) c(n) = sum(k=1, n\2, istotient(k) && istotient(n-k)); a(n) = my(k=1); while(c(eulerphi(k)) != n, k++); k; CROSSREFS Cf. A000010, A002202, A280867, A281687, A281875. Sequence in context: A262433 A116438 A191078 * A117682 A045429 A168164 Adjacent sequences:  A281813 A281814 A281815 * A281817 A281818 A281819 KEYWORD nonn,more AUTHOR Altug Alkan, Jan 30 2017 EXTENSIONS a(0) = 1 prepended by Chai Wah Wu, Feb 03 2017 STATUS approved

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Last modified August 13 04:54 EDT 2020. Contains 336442 sequences. (Running on oeis4.)