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A291932 a(n) is the smallest k such that (n+1)*phi(k) = (n-1)*psi(k). 2
2, 3, 95, 5, 143, 7, 319, 323, 559, 11, 117317, 13, 1007, 899, 1919, 17, 201983, 19, 441283, 1763, 394697, 23, 4031, 5249, 2911, 3239, 23519, 29, 3599, 31, 1796647, 979801, 8159, 5459, 5183, 37, 1550047, 10763, 8639, 41, 2709037, 43, 10207, 9179, 101567, 47, 12218993, 9701, 13199, 10403, 4018073, 53 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

COMMENTS

Least k such that Product_{p|k} (p+1)/(p-1) = (n+1)/(n-1). As a result, all terms are squarefree. - Charles R Greathouse IV, Sep 06 2017

a(102) > 100000000. - Robert G. Wilson v, Sep 08 2017

a(102) = 8759437837. - Giovanni Resta, Sep 11 2017

a(108) > 2550000000. - Robert G. Wilson v, Sep 20 2017

LINKS

Robert G. Wilson v, Table of n, a(n) for n = 2..107

FORMULA

a(p) = p for all primes p.

EXAMPLE

a(4) = 95 = 5*19 because (psi(5*19) + phi(5*19)) / (psi(5*19) - phi(5*19)) = (6*20 + 4*18) / (6*20 - 4*18) = 4 and 95 is the least number with this property.

MAPLE

N:= 10^7: # to get all terms before the first with a(n) > N

M:= nextprime(N):

A:= Vector(M):

R:= proc(n) mul((i[1]+1)/(i[1]-1), i=ifactors(n)[2]) end proc:

for k from 2 to N do

r:= R(k);

n:= (r+1)/(r-1);

if n::integer and n <= M and A[n] = 0 then

  A[n]:= k;

fi

od:

m:=min(select(t -> A[t]=0, [$2..M]))-1:

seq(A[i], i=2..m); # Robert Israel, Sep 06 2017

MATHEMATICA

psi[n_] := If[n < 1, 0, n Sum[ MoebiusMu[d]^2/d, {d, Divisors@ n}]]; f[n_] := Block[{k = 1}, While[(n + 1)*EulerPhi[k] != (n - 1)*psi[k], k++]; k]; Array[f, 52, 2] (* Robert G. Wilson v, Sep 06 2017 *)

PROG

(PARI) a(n)=my(target=2/(n-1)+1, start=n, end=10*n, f); while(1, forfactored(k=start, end, f=k[2][, 1]; if(vecmax(k[2][, 2])==1 && prod(i=1, #f, 2/(f[i]-1)+1)==target, return(k[1]))); start=end+1; end*=2) \\ Charles R Greathouse IV, Sep 06 2017

CROSSREFS

Cf. A000010, A001615.

Sequence in context: A042901 A002983 A118167 * A247652 A132499 A199541

Adjacent sequences:  A291929 A291930 A291931 * A291933 A291934 A291935

KEYWORD

nonn

AUTHOR

Altug Alkan, Sep 06 2017

STATUS

approved

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Last modified April 20 20:26 EDT 2021. Contains 343137 sequences. (Running on oeis4.)