cp's OEIS Frontend

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.

%I A281816 #29 Feb 06 2017 02:22:40
%S A281816 1,3,11,13,23,29,37,41,81,53,67,61,73,97,103,89,109,143,139,113,137,
%T A281816 157,149
%N 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.
%C A281816 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".
%C A281816 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.
%e A281816 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.
%o A281816 (PARI) c(n) = sum(k=1, n\2, istotient(k) && istotient(n-k));
%o A281816 a(n) = my(k=1); while(c(eulerphi(k)) != n, k++); k;
%Y A281816 Cf. A000010, A002202, A280867, A281687, A281875.
%K A281816 nonn,more
%O A281816 0,2
%A A281816 _Altug Alkan_, Jan 30 2017
%E A281816 a(0) = 1 prepended by _Chai Wah Wu_, Feb 03 2017