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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

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*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

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

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Last modified April 10 16:05 EDT 2021. Contains 342845 sequences. (Running on oeis4.)