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A337856
Number of positive integers with n digits that are the product of two integers ending with 6.
12
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.
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
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