

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
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OFFSET

1,4


COMMENTS

See also graph of A045917 ("Goldbach's comet").  Altug Alkan, Jan 30 2017


LINKS

Robert Israel, Table of n, a(n) for n = 1..10000
Altug Alkan, Illustration Of Residue Classes Modulo 6


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*i1]+V[i])/2, i=1..N); # Robert Israel, Jan 27 2017


PROG

(PARI) a(n) = sum(k=1, n, istotient(k) && istotient(2*nk));


CROSSREFS

Cf. A000010, A002202, A045917, A280867.
Sequence in context: A330561 A048688 A092695 * A033270 A285507 A103264
Adjacent sequences: A281684 A281685 A281686 * A281688 A281689 A281690


KEYWORD

nonn,look


AUTHOR

Altug Alkan, Jan 27 2017


STATUS

approved



