A337856 Number of positive integers with n digits that are the product of two integers ending with 6.

0, 2, 20, 230, 2515, 26889, 282211, 2930013, 30196730, 309564822, 3161099901, 32182595954, 326874672928, 3313770788984

Offset

1,2

Comments

a(n) is the number of n-digit numbers in A324297.

Links

Table of n, a(n) for n=1..14.

Index entries for sequences related to digits.

Formula

Conjecture: Lim_{n->infinity} a(n)/a(n-1) = 10.

Prog

(Python)
def A337856(n):
    k, n1, n2, pset = 0, 10**(n-1)//2-18, 10**n//2-18, set()
    while 50*k**2+60*k < n2:
        a, b = divmod(n1-30*k, 50*k+30)
        m = max(k, a+int(b>0))
        r = 50*k*m+30*(k+m)
        while r < n2:
            pset.add(r)
            m += 1
            r += 50*k+30
        k += 1
    return len(pset) # Chai Wah Wu, Sep 26 2021

Crossrefs

Cf. A017341, A324297, A337855, A346509.

Sequence in context: A227337 A127110 A361964 * A296660 A197898 A293471

Adjacent sequences: A337853 A337854 A337855 * A337857 A337858 A337859

Keyword

nonn,base,hard,more

Author

Stefano Spezia, Sep 27 2020

Extensions

a(5) corrected by and a(6)-a(9) from Jinyuan Wang, Oct 01 2020

a(10)-a(13) from Bert Dobbelaere, Oct 20 2020

a(14) from Martin Ehrenstein, Aug 06 2021

Status

approved