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%I #29 Sep 22 2023 02:11:17
%S 1,1,1,4,4,4,4,8,9,9,9,9,9,9,9,16,16,16,16,16,16,16,16,16,25,25,27,27,
%T 27,27,27,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,49,49,49,
%U 49,49,49,49,49,49,49,49,49,49,49,49,64,64,64,64,64,64,64,64,64,64,64
%N Largest prime power <= n that is not prime.
%H Robert Price, <a href="/A167185/b167185.txt">Table of n, a(n) for n = 1..2000</a>
%e For a(14), 10, 12, and 14 are not prime powers, and 11 and 13 are prime powers but they are prime. Since 9 = 3^3 is a prime power, a(14) = 9.
%t Array[SelectFirst[Range[#, 1, -1], Or[And[! PrimeQ@ #, PrimePowerQ@ #], # == 1] &] &, 74] (* _Michael De Vlieger_, Jun 14 2017 *)
%o (PARI) isA025475(n) = (omega(n) == 1 & !isprime(n)) || (n == 1)
%o A167185(n) = {local(m);m=n;while(!isA025475(m),m--);m}
%o (Sage)
%o p = [n for n in (1..81) if (is_prime_power(n) or n == 1) and not is_prime(n)]
%o r = [[p[i]]*(p[i+1] - p[i]) for i in (0..9)]
%o print([y for x in r for y in x]) # _Peter Luschny_, Jun 14 2017
%Y List of nonprime prime powers: A025475.
%Y Next nonprime prime power: A167184.
%Y Previous prime power including primes: A031218.
%K nonn,easy
%O 1,4
%A _Michael B. Porter_, Oct 29 2009
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