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A166684
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Numbers n such that d(n)<4.
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12
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1, 2, 3, 4, 5, 7, 9, 11, 13, 17, 19, 23, 25, 29, 31, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 121, 127, 131, 137, 139, 149, 151, 157, 163, 167, 169, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251
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OFFSET
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1,2
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COMMENTS
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1 together with primes and squares of primes.
Numbers that cannot be written as a product of 2 distinct nonunits. - Peter Munn, May 26 2023
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LINKS
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FORMULA
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MATHEMATICA
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Select[Range[300], DivisorSigma[0, #]<4&] (* or *) Select[With[ {prs = Prime[Range[200]]}, Union[Join[{1}, prs, prs^2]]], #<301&] (* Harvey P. Dale, Jan 04 2012 *)
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PROG
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(Python)
from math import isqrt
from sympy import primepi
def f(x): return n-1+x-primepi(x)-primepi(isqrt(x))
m, k = n, f(n)
while m != k:
m, k = k, f(k)
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CROSSREFS
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A000430 is the main entry for this sequence.
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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