The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A339928 Numbers k such that the removal of all terminating even digits from k! leaves a prime. 0
 6, 7, 9, 10, 43, 138, 1068 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(8) > 1500. If only the terminating zeros are removed, 2 is the only number whose factorial becomes prime. If one also removes 5s at the end, 7 is no longer in the sequence and no numbers below 1500 are added to the sequence. LINKS EXAMPLE 43! = 60415263063373835637355132068513997507264512000000000. After removing all even digits at the end, we are left with 6041526306337383563735513206851399750726451, which is prime. So 43 is a term of this sequence. 27! = 10888869450418352160768000000. After removing all even digits at the end, we are left with 108888694504183521607, which is not prime. So 27 is not a term of this sequence. PROG (PARI) for(n=1, 1500, k=n!; while(!(k%2), k\=10; if(k==0, break)); if(isprime(k), print1(n, ", "))) (Python) from sympy import factorial, isprime def ok(n):     fn = factorial(n)     while fn > 0 and fn%2 == 0: fn //= 10     return fn > 0 and isprime(fn) print(list(filter(ok, range(200)))) # Michael S. Branicky, Jun 07 2021 CROSSREFS Cf. A000142. Sequence in context: A287347 A216361 A216360 * A205877 A081053 A022892 Adjacent sequences:  A339925 A339926 A339927 * A339929 A339930 A339931 KEYWORD nonn,base,more AUTHOR Derek Orr, Dec 23 2020 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 17 19:21 EDT 2022. Contains 353778 sequences. (Running on oeis4.)