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A281687 Number of partitions of 2*n into the sum of two totient numbers (A002202). 4
1, 1, 1, 2, 2, 3, 3, 3, 3, 4, 4, 5, 5, 5, 5, 6, 6, 6, 6, 7, 6, 7, 7, 9, 8, 9, 8, 9, 9, 11, 10, 12, 10, 11, 10, 12, 11, 13, 10, 11, 12, 13, 12, 15, 13, 12, 13, 13, 12, 15, 14, 14, 14, 16, 15, 19, 16, 16, 16, 17, 15, 19, 15, 18, 16, 19, 16, 20, 18, 19, 18, 20, 17, 22, 19, 21, 18, 21, 19, 22 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,4
COMMENTS
See also graph of A045917 ("Goldbach's comet"). - Altug Alkan, Jan 30 2017
LINKS
EXAMPLE
a(6) = 3 because 2 * 6 = 12 = 2 + 10 = 4 + 8 = 6 + 6 and 2, 4, 6, 8, 10 are in A002202.
MAPLE
N:= 1000: V:= Vector(2*N):
V[1]:= 1:
for n from 2 to 2*N by 2 do
if nops(numtheory:-invphi(n))>1 then V[n]:= 1 fi
od:
C:= map(round, SignalProcessing:-Convolution(V, V)):
seq((C[2*i-1]+V[i])/2, i=1..N); # Robert Israel, Jan 27 2017
PROG
(PARI) a(n) = sum(k=1, n, istotient(k) && istotient(2*n-k));
CROSSREFS
Sequence in context: A330561 A048688 A092695 * A033270 A285507 A103264
KEYWORD
nonn,look
AUTHOR
Altug Alkan, Jan 27 2017
STATUS
approved

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Last modified March 28 05:39 EDT 2024. Contains 371235 sequences. (Running on oeis4.)