A281816 - OEIS

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, 3, 11, 13, 23, 29, 37, 41, 81, 53, 67, 61, 73, 97, 103, 89, 109, 143, 139, 113, 137, 157, 149

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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

Table of n, a(n) for n=0..22.

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