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A336485 The least positive integer k for which there exist primes p, q, r such that phi(p*q*s^k) = phi(r*s^(2k+1)) and sigma(p*q*s^k) = sigma(r*s^(2k+1)), where s is the n-th prime. 1
1, 1, 1, 1, 17, 3, 29, 4, 4, 4, 1, 5, 4, 1, 20, 32, 2, 38, 12, 29, 9, 4, 26, 20, 8, 14, 2, 14, 8, 41, 4 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,5

COMMENTS

This sequence is an inversion of A336486.

A236255 contains the primes s = prime(m) for which a(m) = 1.

Let "a and b are similar" mean that for positive integers a and b we have phi(a) = phi(b), tau(a) = tau(b) and sigma(a) = sigma(b).

Conjecture: For each prime s there are infinitely many positive integers k such that numbers p*q*s^k and r*s^(2k+1) are similar for some primes p, q, r.

LINKS

Table of n, a(n) for n=1..31.

EXAMPLE

a(6) = 3 because:

1. For the 6th prime, s = 13, k = 3 and with primes p = 62807837, q = 57149, r = 125672849 we have phi(p*q*s^k) = phi(r*s^(2k+1)) and sigma(p*q*s^k) = sigma(r*s^(2k+1)).

2. There is no such equality for s = 13 and k less than 3.

MAPLE

with(NumberTheory):

SK := []; for ii to 31 do s := ithprime(ii); tf := false;

for k do c := 2*s^(k+1)+1; cc := (c^2-1)*(1/2); Q := Divisors(cc);

for d in Q do q := d+c; if isprime(q) then p := c+cc/(q-c); if p < q then break end if;

if isprime(p) then r := 2*(p+q)-c; if isprime(r) then print([s, [p, q], r], k); SK := [op(SK), [s, k]]; tf := true; break end if end if end if end do;

if tf then break end if end do end do; SK

PROG

(PARI) is(t, u, x, y) = ispseudoprime(t*x+1) && ispseudoprime(u*y/t+1) && ispseudoprime(x*y+1);

a(n) = {my(s=prime(n), t, u); for(k=1, oo, for(i=0, 1+k\2, t=s^i; fordiv(2*(1+u=s^(k+1)), d, if(is(t, u, 2*u/t+d, 2*t+(2*u+2)/d) || is(t, u, 2*u/t-d, 2*t-(2*u+2)/d), return(k))))); } \\ Jinyuan Wang, Sep 30 2020

CROSSREFS

Cf. A000005, A000010, A000203, A134922, A336486.

Sequence in context: A089570 A040282 A139728 * A317317 A212602 A174380

Adjacent sequences:  A336482 A336483 A336484 * A336486 A336487 A336488

KEYWORD

nonn,more

AUTHOR

Vladimir Letsko, Jul 23 2020

STATUS

approved

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Last modified July 31 04:50 EDT 2021. Contains 346367 sequences. (Running on oeis4.)