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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
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.
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
KEYWORD
nonn,more
AUTHOR
Altug Alkan, Jan 30 2017
EXTENSIONS
a(0) = 1 prepended by Chai Wah Wu, Feb 03 2017
STATUS
approved