A280867 Least k such that k is the sum of two totient numbers (A002202) in exactly n ways, or 0 if no such k exists.

2, 8, 12, 20, 24, 32, 40, 50, 48, 62, 60, 64, 76, 102, 88, 108, 120, 128, 112, 136, 152, 148, 186, 168, 180, 192, 184, 216, 252, 236, 208, 220, 244, 232, 308, 268, 280, 292, 336, 304, 368, 328, 384, 364, 352, 408, 440, 376, 432, 400, 436, 388, 448, 492, 460, 484, 472, 548

Offset

1,1

Comments

Least k such that A281687(k/2) = n, or 0 if no such k exists.

See also graph of A001172 for similar points.

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Example

a(4) = 20 because 20 = 2 + 18 = 4 + 16 = 8 + 12 = 10 + 10; 2, 4, 8, 10, 12, 16, 18 are in A002202 and 20 is the least number with this property.

Prog

(PARI) a281687(n) = sum(k=1, n, istotient(k) && istotient(2*n-k));
a(n) = my(k=1); while(a281687(k) != n, k++); 2*k;
(PARI) do(n)=my(u=select(istotient, [1..n]), v=vector(n), t); for(i=1, #u, for(j=i, #u, t=u[i]+u[j]; if(t>n, break); v[t]++)); t=vector(vecmax(v)); for(i=1, #v, if(v[i] && t[v[i]]==0, t[v[i]]=i)); for(i=1, #t, if(t[i]==0, return(t[1..i-1]))); t \\ Charles R Greathouse IV, Jan 29 2017

Crossrefs

Cf. A000010, A001172, A002202, A281687.

Sequence in context: A099418 A108987 A035405 * A111058 A063622 A162152

Adjacent sequences: A280864 A280865 A280866 * A280868 A280869 A280870

Keyword

nonn

Author

Altug Alkan, Jan 28 2017

Status

approved