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A181830 The number of positive integers <= n that are strongly prime to n. 10
0, 1, 0, 0, 0, 1, 0, 2, 2, 2, 1, 6, 2, 6, 4, 4, 4, 11, 4, 12, 6, 6, 6, 18, 6, 12, 9, 14, 8, 22, 6, 22, 14, 14, 12, 20, 8, 27, 16, 20, 12, 32, 10, 34, 18, 18, 16, 42, 14, 32, 17, 26, 20, 46, 16, 32, 20, 28, 24, 54, 14, 48, 28, 32, 26, 41, 16 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,8

COMMENTS

k is strongly prime to n iff k is relatively prime to n and k does not divide n-1.

a(n) = phi(n) - tau(n-1) if n > 0 and a(0) = 0.

Here phi(n) = A000010(n) and tau(n) = A000005(n).

LINKS

Table of n, a(n) for n=0..66.

Peter Luschny, Strong coprimality.

EXAMPLE

a(11) = card({1,2,3,4,5,6,7,8,9,10} - {1,2,5,10}) = card({3,4,6,7,8,9}) = 6.

MAPLE

with(numtheory):

A181830 := n -> `if`(n=0, 0, phi(n)-tau(n-1)):

StrongCoprimes := n -> select(k->igcd(k, n)=1, {$1..n}) minus divisors(n-1):

A181830a := n -> nops(StrongCoprimes(n)):

MATHEMATICA

a[0]=0; a[1]=1; a[n_ /; n > 1] := Select[Range[n], CoprimeQ[#, n] && !Divisible[n-1, #] &] // Length; Table[a[n], {n, 0, 66}] (* Jean-Fran├žois Alcover, Jun 26 2013 *)

PROG

(PARI) a(n)=if(n, eulerphi(n)-numdiv(n), 0) \\ Charles R Greathouse IV, Jun 26 2013

CROSSREFS

Cf. A181831, A181832, A181833, A181834, A181835, A181836, A000010, A000005.

Sequence in context: A184242 A109978 A114293 * A086612 A128207 A180958

Adjacent sequences:  A181827 A181828 A181829 * A181831 A181832 A181833

KEYWORD

nonn,easy

AUTHOR

Peter Luschny, Nov 17 2010

STATUS

approved

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Last modified October 20 21:27 EDT 2014. Contains 248371 sequences.