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A350654 Smallest k such that A349949(k) = n, or -1 if no such k exists.

%I A350654 #25 Jan 14 2022 07:41:58
%S A350654 2,3,8,15,63,120,440,945,2079,4095,21735,98175,133056,395199,338625,
%T A350654 1890945,3501576,8390304,35820225,126775935,149848335,879207616,
%U A350654 302464800
%N A350654 Smallest k such that A349949(k) = n, or -1 if no such k exists.
%C A350654 a(25) = 879207615. - _Chai Wah Wu_, Jan 13 2022
%o A350654 (PARI) f(n) = my(sd=setunion(divisors(n-1), divisors(n+1))); sumdiv(n, d, (vecsearch(sd, d-1)>0) || (vecsearch(sd, d+1)>0)); \\ A349949
%o A350654 a(n) = my(k=2); while (f(k) != n, k++); k; \\ _Michel Marcus_, Jan 10 2022
%o A350654 (Python)
%o A350654 from itertools import count
%o A350654 from sympy import divisors
%o A350654 def A350654(n):
%o A350654     for m in count(2):
%o A350654         c = 0
%o A350654         for d in divisors(m,generator=True):
%o A350654             if not (((m-1) % (d-1) if d > 1 else True) and (m-1) % (d+1) and ((m+1) % (d-1) if d > 1 else True) and (m+1) % (d+1)):
%o A350654                 c += 1
%o A350654                 if c > n:
%o A350654                     break
%o A350654         if c == n:
%o A350654             return m # _Chai Wah Wu_, Jan 12 2022
%Y A350654 Cf, A000005, A349949.
%K A350654 nonn,more
%O A350654 1,1
%A A350654 _Tejo Vrush_, Jan 09 2022
%E A350654 a(11)-a(19) from _Jinyuan Wang_, Jan 10 2022
%E A350654 Escape clause value changed to -1 by _N. J. A. Sloane_, Jan 12 2022
%E A350654 a(20)-a(21) from _Chai Wah Wu_, Jan 12 2022
%E A350654 a(22)-a(23) from _Chai Wah Wu_, Jan 13 2022