A292874 Least k such that A039654(k) = prime(n).

2, 3, 5, 7, 4, 13, 10, 19, 8, 29, 21, 37, 20, 43, 22, 34, 18, 61, 67, 9, 73, 57, 44, 40, 52, 101, 63, 85, 109, 74, 93, 86, 137, 76, 149, 91, 157, 163, 32, 173, 88, 117, 68, 193, 197, 199, 211, 84, 80, 229, 36, 104, 241, 96, 257, 102, 136

Offset: 1

Author

M. F. Hasler, Sep 25 2017

Keywords

• nonn

Comments

We have A039653(k) >= k, thus also A039654(k) >= k, with equality if k is prime, therefore a(n) <= prime(n), and the largest k for which A039654(k) = prime(n) is always k = prime(n).

Crossrefs

Cf. A039653, A039654, A177343, A292112, A292113.

Programs

PARI

a039654(n) = if(n<2, 0, my(m); while((m=sigma(n)-1)!=n, n=m); n);
a(n) = my(k=0, p=prime(n)); while(a039654(k) != p, k++); k; \\ Michel Marcus, Sep 26 2017