cp's OEIS Frontend

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.

%I A280867 #33 Jan 30 2017 11:51:00
%S A280867 2,8,12,20,24,32,40,50,48,62,60,64,76,102,88,108,120,128,112,136,152,
%T A280867 148,186,168,180,192,184,216,252,236,208,220,244,232,308,268,280,292,
%U A280867 336,304,368,328,384,364,352,408,440,376,432,400,436,388,448,492,460,484,472,548
%N 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.
%C A280867 Least k such that A281687(k/2) = n, or 0 if no such k exists.
%C A280867 See also graph of A001172 for similar points.
%H A280867 Charles R Greathouse IV, <a href="/A280867/b280867.txt">Table of n, a(n) for n = 1..10000</a>
%e A280867 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.
%o A280867 (PARI) a281687(n) = sum(k=1, n, istotient(k) && istotient(2*n-k));
%o A280867 a(n) = my(k=1); while(a281687(k) != n, k++); 2*k;
%o A280867 (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
%Y A280867 Cf. A000010, A001172, A002202, A281687.
%K A280867 nonn
%O A280867 1,1
%A A280867 _Altug Alkan_, Jan 28 2017