Hi,

I am trying to fit a robust regression using code from https://jrnold.github.io/bayesian_notes/robust-regression.html, however I am getting sampling errors below. Is this an error with my installation or the model please. Thank you.

```
# SAMPLING FOR MODEL '92be42a71d12c73fccc7fa1ae9581a37' NOW (CHAIN 1).
# Chain 1: Rejecting initial value:
# Chain 1: Error evaluating the log probability at the initial value.
# Chain 1: Exception: gamma_lpdf: Shape parameter is -0.39896, but must be > 0! (in 'model20e226d00e0a_92be42a71d12c73fccc7fa1ae9581a37' at line 35)
#
# Chain 1: Rejecting initial value:
# Chain 1: Error evaluating the log probability at the initial value.
# Chain 1: Exception: gamma_lpdf: Shape parameter is -0.188494, but must be > 0! (in 'model20e226d00e0a_92be42a71d12c73fccc7fa1ae9581a37' at line 35)
#
# Chain 1: Rejecting initial value:
# Chain 1: Log probability evaluates to log(0), i.e. negative infinity.
# Chain 1: Stan can't start sampling from this initial value.
# Chain 1: Rejecting initial value:
# Chain 1: Error evaluating the log probability at the initial value.
# Chain 1: Exception: exponential_lpdf: Random variable is -0.239801, but must be >= 0! (in 'model20e226d00e0a_92be42a71d12c73fccc7fa1ae9581a37' at line 32)
#
...
...
...
```

This is code to reproduce

```
txt=
'
// Linear Model with Student-t Errors
data {
int N;
vector[N] y;
int K;
matrix [N, K] X;
real scale_alpha;
vector[K] scale_beta;
real loc_sigma;
}
parameters {
real alpha;
vector[K] beta;
real sigma;
vector[N] inv_lambda2;
real nu;
}
transformed parameters {
vector[N] mu;
vector[N] omega;
for (n in 1:N) {
omega[n] = sigma / sqrt(inv_lambda2[n]);
}
mu = alpha + X * beta;
}
model {
real half_nu;
alpha ~ normal(0.0, scale_alpha);
beta ~ normal(0.0, scale_beta);
sigma ~ exponential(loc_sigma);
nu ~ gamma(2, 0.1);
half_nu = 0.5 * nu;
inv_lambda2 ~ gamma(half_nu, half_nu);
y ~ normal(mu, sigma);
}
'
stan(model_code=txt,
data = list(y=mtcars$mpg, X=cbind(mtcars$wt, mtcars$disp),
K=2, N=nrow(mtcars),
scale_alpha=10, scale_beta=c(10, 10), loc_sigma=1),
chains=1, seed=1)
```

My sessionInfo()

```
R version 3.6.2 (2019-12-12)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 19.04
Matrix products: default
BLAS: /usr/lib/x86_64-linux-gnu/openblas/libblas.so.3
LAPACK: /usr/lib/x86_64-linux-gnu/libopenblasp-r0.3.5.so
locale:
[1] LC_CTYPE=en_GB.UTF-8 LC_NUMERIC=C LC_TIME=en_GB.UTF-8 LC_COLLATE=en_GB.UTF-8 LC_MONETARY=en_GB.UTF-8
[6] LC_MESSAGES=en_GB.UTF-8 LC_PAPER=en_GB.UTF-8 LC_NAME=C LC_ADDRESS=C LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_GB.UTF-8 LC_IDENTIFICATION=C
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] rstan_2.19.2 ggplot2_3.2.1 StanHeaders_2.19.0
loaded via a namespace (and not attached):
[1] Rcpp_1.0.2 rstudioapi_0.10 magrittr_1.5 tidyselect_0.2.5 munsell_0.5.0 colorspace_1.4-1 R6_2.4.0
[8] rlang_0.4.0 dplyr_0.8.3 tools_3.6.2 parallel_3.6.2 pkgbuild_1.0.5 grid_3.6.2 gtable_0.3.0
[15] loo_2.1.0 cli_1.1.0 withr_2.1.2 matrixStats_0.55.0 lazyeval_0.2.2 assertthat_0.2.1 tibble_2.1.3
[22] crayon_1.3.4 processx_3.4.1 gridExtra_2.3 purrr_0.3.2 callr_3.3.1 codetools_0.2-16 ps_1.3.0
[29] inline_0.3.15 glue_1.3.1 compiler_3.6.2 pillar_1.4.2 prettyunits_1.0.2 scales_1.0.0 stats4_3.6.2
[36] pkgconfig_2.0.2
```