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A358239
Numbers k such that the aliquot sequence of 2^k ends with the prime 3.
2
2, 4, 55, 164, 305, 317
OFFSET
1,1
LINKS
FORMULA
Define s(i) = sigma(i) - i = A000203(i) - i. Then k is a term of this sequence if the aliquot sequence obtained by repeatedly applying the mapping i->s(i) taking as initial value 2^k terminates in the prime 3.
EXAMPLE
a(3)=55 because the aliquot sequence that starts with the integer 2^55 ends with the prime number 3 and there are only two smaller powers of 2 that do the same: 2^2 and 2^4.
PROG
(PARI) f(n) = if (n==1, return(2)); my(list = List(), s=2^n); for (i=1, oo, s = sigma(s) - s; if (#select(x->(x==s), list), return(0)); if (isprime(s), return (s)); listput(list, s); );
isok(m) = f(m) == 3; \\ Michel Marcus, Nov 05 2022
CROSSREFS
Sequence in context: A005274 A121234 A084574 * A087621 A062784 A318642
KEYWORD
nonn,hard,more
AUTHOR
Jean Luc Garambois, Nov 04 2022
STATUS
approved