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A215199 Smallest number k such that k and k+1 are both of the form p*q^n where p and q are distinct primes. 5
14, 44, 135, 2511, 8991, 29888, 916352, 12393728, 155161088, 2200933376, 6856828928, 689278976, 481758175232, 3684603215871, 35419114668032, 2035980763136, 174123685117952, 9399153082499072, 19047348965998592, 203368956137832447, 24217192574746623, 2503092614937444351 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(15) <= 35419114668032. - Donovan Johnson, Aug 22 2012

If k is a term such that k = p*q^n and k+1 = r*s^n, where p,q,r,s are primes, then clearly q != s. Conjecture: q and s are either 2 or 3 for all terms. - Chai Wah Wu, Mar 10 2019

Since q^n and s^n are coprime, the Chinese Remainder Theorem can be used to find candidate terms to test, i.e., numbers k such that k+1 == 0 (mod s^n) and k+1 == 1 (mod q^n) (see Python code). - Chai Wah Wu, Mar 12 2019

From David A. Corneth, Mar 13 2019: (Start)

Conjecture: Let 1 <= D < 2^n be the denominator of N/D of (3/2)^n. Without loss of generality, if the conjecture above holds that (q, s) = (2, 3) then r = D + k*2^n for some n.

Example: for n = 100, we have the continued fraction of (3/2)^100 to be 406561177535215237, 2, 1, 1, 14, 9, 1, 1, 2, 2, 1, 4, 1, 2, 6, 5, 1, 195, 3, 26, 39, 6, 1, 1, 1, 2, 7, 1, 4, 2, 1, 11, 1, 25, 6, 1, 4, 3, 2, 112, 1, 2, 1, 3, 1, 3, 4, 8, 1, 1, 12, 2, 1, 3, 2, 2 from which we compute D = 519502503658624787456021964081. We find r = 1100840223501761745286594404230449 = D + 868 * 2^100 giving a(100) + 1 = r*3^100. - David A. Corneth, Mar 13 2019

LINKS

Chai Wah Wu, Table of n, a(n) for n = 1..1279 (n = 25..32 from David A. Corneth)

EXAMPLE

a(3) = 135 because 135 = 5*3^3 and 136 = 17*2^3;

a(4) = 2511 because 2511 = 31*3^4 and 2512 = 157*2^4.

MAPLE

psig := proc(n)

    local s, p ;

    s := [] ;

    for p in ifactors(n)[2] do

        s := [op(s), op(2, p)] ;

    end do:

    sort(s) ;

end proc:

A215199 := proc(n)

    local slim, smi, sma, ca, qi, q, p, k ;

    for slim from 0 do

        smi := slim*1000 ;

        sma := (slim+1)*1000 ;

        ca := sma ;

        q := 2 ;

        for qi from 1 do

            p := nextprime(floor(smi/q^n)-1) ;

            while p*q^n < sma do

                if p <> q then

                    k := p*q^n ;

                    if psig(k+1) = [1, n] then

                        ca := min(ca, k) ;

                    end if;

                end if;

                p := nextprime(p) ;

            end do:

            if q^n >= sma then

                break;

            end if;

            q := nextprime(q) ;

        end do:

        if ca < sma then

            return ca ;

        end if;

    end do:

end proc:

for n from 1 do

    print(A215199(n)) ;

end do; # R. J. Mathar, Aug 07 2012

PROG

(Python)

from sympy import isprime, nextprime

from sympy.ntheory.modular import crt

def A215199(n):

    l = len(str(3**n))-1

    l10, result = 10**l, 2*10**l

    while result >= 2*l10:

        l += 1

        l102, result = l10, 20*l10

        l10 *= 10

        q, qn = 2, 2**n

        while qn <= l10:

            s, sn = 2, 2**n

            while sn <= l10:

                if s != q:

                    a, b = crt([qn, sn], [0, 1])

                    if a <= l102:

                        a = b*(l102//b) + a

                    while a < l10:

                        p, t = a//qn, (a-1)//sn

                        if p != q and t != s and isprime(p) and isprime(t):

                            result = min(result, a-1)

                        a += b

                s = nextprime(s)

                sn = s**n

            q = nextprime(q)

            qn = q**n

    return result # Chai Wah Wu, Mar 12 2019

CROSSREFS

Cf. A074172, A215173, A215197, A215198.

Sequence in context: A189807 A009942 A031130 * A216258 A064348 A206215

Adjacent sequences:  A215196 A215197 A215198 * A215200 A215201 A215202

KEYWORD

nonn,hard

AUTHOR

Michel Lagneau, Aug 05 2012

EXTENSIONS

a(10)-a(14) from Donovan Johnson, Aug 22 2012

a(15)-a(17) from Chai Wah Wu, Mar 09 2019

a(18)-a(22) from Chai Wah Wu, Mar 10 2019

STATUS

approved

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Last modified September 20 14:44 EDT 2021. Contains 347586 sequences. (Running on oeis4.)